"What if she doesn't like dinosaurs?" is just as silly as saying "what if she doesn't like pink?"
We all decorate our kids' rooms how WE want to. If the kid decides he/she doesn't like it when he/she gets older, it's an easy change. Do your dinos and ignore your judgy MIL.
This. I am doing elephants. Not because I think she'll like elephants, but because I like elephants. The first year or two of her life she certainly won't be voicing her opinion about it, and chances are, because she grew up with dinosaurs, she WILL like dinosaurs. Or, maybe she won't, but still, once she is old enough to make her own opinion about it, it's easy enough to change. I think it's awesome and I have seen some cute girly dinosaur things! Ignore MIL!!!