Question: Why did Mrs. Incredible look at herself in the mirror? I never did understand that.
Answer: She was noticing her larger hips/butt thanks to the skin-tight super suit. As with a lot of people, she gained weight as she got older, and she was no longer a superhero, so she didn't get the exercise she had in her youth.
Answer: She sees her reflection and notices her thighs and rear-end look a little big, before letting out a slightly disappointed sigh. It's there to indicate that she has minor body-insecurity issues, much as Bob did earlier in the film. She hasn't been a super in a while and has had kids in the meantime, so she's put on a little weight... it's a nice moment to remind us that despite being a superhero on the surface, she's also a normal person with normal worries deep down inside.
TedStixon