Yours is a really interesting comment, and I'm glad you posted it.
Our daughter was a CHUNK when she was born and until about 4 months ago - she's now a little past 2. I started calling her "Munchkin" as a nickname so it was a positive name instead of "ChunkaPoo" that we were calling her.
I, too, want to be known personally and professionally for my integrity, compassion, passion, ethics, intellect, and I want to instill into my children the importance of being a genuinely good person.
But, I also had a mother that would pick out my flaws and tell me that I wasn't as pretty as one of my sisters, that my thighs were too large for my frame, that I was short waisted, etc. Even now, I have a very poor body image that isn't healthy.
So, I tell my daughter all the time how gorgeous she is. I don't even want her to doubt herself in the most overt ways that most people judge us on.
I work, though, in an industry filled with "pretty people". It was a model (no pun intended) adopted in the 1990s that sending a gorgeous/handsome, young professional into a doctor's office would help generate conversation and time. I work in pharmaceutical sales/marketing. I have seen many, many women wearing completely inappropriate clothing and actually being kicked out of offices for not taking themselves seriously enough.
When it comes to your MIL, I'd not say something (my opinion) until it gets out of hand. If all she focuses on his her appearance and not her giftedness in academics, athletics, music, philanthropy, etc. I'd have the conversation, but the best you and your husband can do is to focus on the important attributes on your end and make her as well-rounded as possible.
Good luck!