The spackle concept
I had lots of annoyances with my first butterfly because the mortar I spread over my hardware cloth wasn't perfectly even. Ironically, I haven't had this similar problem as I'm working on Mr. Sea. I think because he's a larger piece, he was easier to really keep smooth in the mortar stage. That and, because he's a larger piece, I'm finding it easier to adjust for slight tilts in some of the tiles.
Anyway, I'm trying something a little different on my next butterfly and I hope it helps. I took a putty knife and some thinset and basically "spackled" the butterfly wings. I'm letting it dry good, and then I'll lightly sand over it to get the surface even more smooth.
I have no idea whether or not this will help, but thought I'd share the idea!
I'll keep you posted!