Yes. But, considering that you are teasing targets that won't turn into paranoid egomaniacs anytime soon, and that you show other interests other than assault them, I don't think it's something you should worry about.
Actually we've seen much more of DT than her being just a simple bully.
In The Show Stoppers we see her and Silver Spoon took part in the talent show.
In Return of Harmony we see her and Silver Spoon walking alongside Cheerilee, not bored, but seemingly taking an interest in their teacher's lesson. Then when the CMC started fighting she just looked plain annoyed.
In Family Appreciation Day she was filled with pride and joy at her father's speech, and her teasing AB about Granny Smith was perfectly believeable. I mean, what kid wouldn't laugh when they saw their classmate's grandparent wearing a beard of bees or just genuinly embarassing their classmate in front of everybody?
In Hearts and Hooves Day she had a perfect chance to make fun of the card the CMC gave to Cheerilee, but she ignored it (despite it being the elephant in the room) in favour of socialising with the other ponies and taking part in the fun and games. See that genuine look of excitment on her face at Pin the Tail on the Pony?
In Ponyville Confidential she cared more about building up the school's paper than she did tormenting the CMC. In fact, until they said that they wanted out she was being nice to them, offering them words of praise and encouragement.
In One Bad Seed, she attended the parade, but not once did she do anything to the CMC, despite having plenty of chance to do so. She simply went there to watch, like everypony else. And then she took time out of her day to see her friend off at the train station. You can hardly blame her for that crack about the CMC, since as far as she was concerened Babs disliked the CMC and making cracks about them was just one of the ways they had some 'harmless' fun.
Now let's look at her 'bullying' moments:
1) The infamous blank flank name-calling in Call of the Cutie. Well, we know that cutie marks are a big deal, and when a rich, spoiled kid has something that's a big deal, they show it off to kids that don't have it. Simple as. Fact is, she still invited AB, Scoots, and Sweetie Belle to her party. She didn't go out of her way to see if AB was there, she just happened to find her. And she was hardly the only pony who found the 'blank flank' term to be funny. Hell, if even the adults found it funny, that automatically says something about ponies in Ponyville in general.
2) Almost a whole season later, DT calls AB out on her fake cutie mark in Cutie Pox. This isn't even what I'd call bullying, but for the sake of being thorough, I'll include it. She simply accussed AB of being a faker, and when she was wrong, she (begrudingly) accepted it, and even joined in with the class in celebrating AB's new cutie mark... for some reason. Not sure why AB's cutie mark in particular was so special that they cancelled a lesson for it.
4) Family Appreciation Day. Again, a kid laughing at a classmate's embarassing relative isn't a new thing. They do it, because to them it's funny. And let's face it, Granny Smith /is/ funny. Her last crack about Granny Smith being a 'kooky old lady' was not only justified given Granny Smith licking chalk off of a chalkboard as she said it, but to me seemed more a defence mechanism when AB decided to taunt DT about Granny Smith's story.
5) Ponyville Confidential - The blackmail thing is really the only seriously bad thing DT's done to date, and she was punished for that. She recieved discipline for it, so it shouldn't be something to linger on, but I will say this: it didn't seem to be done out of malicious intent, simply to ensure the newspaper was suscsessful.
6) One Bad Seed - She made a crack about the CMC being losers... and that was it, until Babs joined her side. Aftet that, she tagged along with Babs, who showed her what /real/ bullying was. For the most part, DT and SS did nothing, whilst Babs /really/ showed the Crusaders hell.
So throughout the series thus far, she's had six 'bullying' moments, most of which are questionable the extent to which they're even bullying rather than simply teasing, and six moments where she's shown to just be an ordinary filly with interest outside of bothering the CMC, and where she's been shown to be perfectly able to get along with other ponies.
You'll also notice, despite being commonly associated with calling the CMC 'blank flanks', she's only ever done that in one episode. She even made friends with a blank flank in One Bad Apple, showing she doesn't have a problem with blank flanks, and she also doesn't just re-use the same old material over and over again.
And if you really think about it, the CMC are a group of loud, overly-excited fillies who like to cause messes and create mayhem wherever they go, from using magic potions on ponies without their consent, to destruction of public property, to food poisoning, to messing up their families' businesses (meanwhile, DT and SS are likely the sole reason Carousal Botique has been in business as long as it has in a town where ponies walk around naked all of the time), to just generally being so annoying a lot of the time. It's kinda natural for rich, spoiled kids like DT and SS to pick out the CMC as targets. Doesn't mean it's all that they do, or that's all they'll ever do, or that it even defines who they are.
Considering all the crappy behavior this character has displayed in the series so far (including the upcoming thing with Scootaloo) I would have to say yes, Diamond Tiara is a bully that hasn't learned a thing from her experiences so far.
I for one, hope that the show can give this character some backstory and development and not stay as the most hated cliche. The only bully characters I feel as real characters are buford from phineas and ferb, and nelson from the simpsons. Please guys I implore you, give me a reason to care about dimond tiara, don't keep her as a tool.
From TheMasterOfDespair, founder of the Diamond Cutters:
A lot of Bronies seem to think you should only love and tolerate the ones that deserve it. If you already love someone, then you obviously tolerate them. Tolerance is the main word in this phrase. Even if you strongly dislike them or flat out hate them, you should still show a little compassion. Who knows? Over time you may fall in love with them. This group will stand up for Diamond Tiara, even if we stand alone.
Even Babs had a back story explaining her actions. Sunset Shimmer too, and hers was REALLY weak so she's on my hate list too.
Diamond Tiara's back story is that she just enjoys bullying other ponies. She has nothing redeeming about her at all. Villains aren't born, they're made. Usually something TERRIBLE happens that turns a character into a monster, DT has EVERYTHING going for her, with no agenda other than to hurt others because she thinks its fun.
She wears a freakin' tiara on her head. Yeah she needs to be taken out back and have the ever living sh** kicked out of her. In the Season 4 previews she makes fun of handicapped kids, she needs to die.