No one is going to think your costume is ugly because you don't have large breasts. :] Don't worry.
I have fairly small breasts myself (the equivalent volume of a 34A or 34B but in a smaller band size -- 28D or 28DD) and have had no problems doing bunnysuit cosplays.
Though, I have to wonder how getting cleavage hurts your chest. What method are you using? If you are doing something like the drag queen tape method or pushing your breasts together with your hands, you may be pushing too far. Your breasts may be too small, dense, or far apart for that "buttcrack" style cleavage you usually see. If that's the case, I'd recommend doing a bit of contour shading with makeup, still wearing pushup padding, and going with it. Padding in and of itself shouldn't hurt you in any way.
I'd recommend adding a bit of padding to the suit, mostly just to keep it smooth in front and to create the illusion of a bit more curve. You don't have to make the bust larger than your actual bust and pad out to get overtly larger breasts, but just those molded foam push-up bra cups from the fabric store should suffice. This will also help prevent the ends of your boning from poking you, if you choose to end the boning underneath the bust (I usually do for the center and side front seams -- side seam boning will NEED to go above bust level). Adding bra cups to the structure of the suit will keep the structure of the suit in place and smooth better than if you were to go without any sort of cup in there (and just the fabric).
Here's a good shot of how a bunnysuit looks on me:
http://68.media.tumblr.com/5127b0a38b635fe825e40e7765a1215f/tumblr_ohut3amOJA1s5pg8ho1_1280.jpgIf you need any tips on how to make the suit, let me know. I've done two so far, and they get easier each time.