Is it a real tuck-in tee shirt or was it a half short with her belly showing?
There's a time and place for everything - wearing a tee shirt that is sexually suggestive by it's context beyond the actually cut of the tee infront of your boyfriend's mother is in bad taste.
What her shirt said to your mom is "I fuck your boyfriend and I do it well". So I have to argree with your mom.
If you're talking about going to the gym, hanging at the beach or at a friend's house (in your late teens and 20's) I see nothing wrong with it. Providing a woman's breats are not falling out of her bra and neckline and her naked nipples aren't showing because the shirt is so short and she chose not to wear a bra.
This is just my opinion and I am fully aware that it is old school and harsh but that is just how I feel about it. SO since I am aware of my feeling I try not to judge someone by their first appearance.