I don't see how it could be unless it impedes your movement. Some dancers and audience members will like a big booty and some won't, but it won't physically stop you from dancing. Your skirt may be shorter in the back, though.
Too big for dancing? In what way? Are you wondering if it's possible to have a butt so big that it's physically impossible or difficult or painful to dance? Or are you wondering if people find it aesthetically pleasing to watch dancers whose butts are disproportionately huge?
If it's the first, I don't think that really happens... I mean, it's possible that someone is so very obese that they can not perform very much physical activity, but that has more to do with the obesity in general than it does the size of the person's butt specifically.
If you are talking about aesthetics, I think that belly dancers have a variety of body types and there are people out there who can appreciate the majority of them. Generally speaking in belly dance we favor wide hips (which often comes with big butts), thin waists and at least a little bit of fat around the midsection but that doesn't mean people with other body types are not appreciated, and it does not mean there aren't people out there who don't dislike wide hips and big butts. This is just a very general generalization.
But then if you are talking about dance in general and not just belly dance, you will find that in some dances (ballet comes to mind), having a big butt is not very desired at all. Ballerinas tend to have really lean, strong bodies with very little fat... and most of the famous ones do not have very prominent glutes. Though male ballet dancers usually have huge butts... all muscle of course!
Strange to hear that in the context of belly dance, but people have different tastes. I've personally never heard any negative comments about belly dancers with big butts (again with different dances it's another story), but it depends on the individual and it depends on the region. I guess in places where big butts aren't appreciated, they may not be appreciated in belly dance either.
Well, if you can stand and walk, why should a big butt hinder dancing ?
Of course one may say a big butt equals overweight and there the question of fitness where not all people with big butts are overweight and many are certainly fit enough to propel themselves, but for those who are unfit, they face exactly the same challenges as an unfit bean pole and it is just a case of working hard to get fit if fitness is desired and well dancing is hard work, fitness is required and there perhaps another clue....
It might be given as a society we are becoming lazier, a big butt might just be an excuse not to exercise or sadly worse; to be seen to be exercising as it does occur there are many in society who like to put others down because they don't fit the media motivated ideal physique.
When I first started dancing, my teacher taught me about the slight tuck. Her butt basically disappeared and well, mine still stuck out. I felt, at the time, that it was a bit of a hindrance. Now, it's just my big butt sticking out and making me roll the front part of my skirts down (I get them to fit my butt and then roll the front down and pin it). I'm not afraid to show it off, give me a nice trumpet skirt and I'll rock it. So that's been my experience with a big butt. It hasn't hindered my dancing, I'm obese but I'm very mobile (at one point I was working 2 jobs, dancing 3 classes/rehearsals a week).