I don't think you're being an @ss, I think your MIL is being presumptuous. If you and your husband agreed on adding your mother's middle name, then continue as planned. She can't do anything about it.
As I see it, your only choice now is whether to tell your MIL, point blank, that you will not be using her names and deal with her being butthurt, or just ignore her and deal with the persistent suggestions. I'm thinking that either way she's going to have something to say. Something about pregnancy does this to people (especially grandmas).