JessicaK:
Goodness. I've been, erm, labelled thusly a couple of times and always assumed it was an insult. Perhaps in the right crowd it's really not.
The "right crowd" would be people with 'xy' chromosome pairs, I'm figuring.
If a male says that you have a "bubblebutt," I'd estimate that 98% of the time you should feel flattered rather than insulted.
Now, what an envious female might mean in applying the term to your derriere, that's another issue...

--C.K.