I know how you feel. Comments are great sometimes, but other times you just wish you were like a caterpillar and could crawl into a chrysalis and just pop out when you're done.
It's really hard to say something because people can get defensive (since they're essentially giving what they believe are compliments) and I don't know how your MIL might react.
I did mostly grinning and bearing it when my family or friends commented EVERY. SINGLE. TIME. I saw them. I too started to dread seeing them or even losing weight because it eventually got to the point where people would start asking me questions I didn't want to answer (how much more weight I wanted to lose, what did I weigh when I started, etc.)...so I started getting sarcastic. When people said: "OH MY GOD YOU LOST WEIGHT" I would say something like: "woah! I had no idea! I wondered why my clothes were big on me!"
Luckily a majority of my crew would just say: "You look great!" and then move on to whatever we were talking about.