I used to wear a bra top at the gym, but I wouldn't do that now. I don't really have a muffin top, and my stomach is pretty flat for an old lady, but it's not like the body of a 20 year old! I always felt like I was in public, wearing my underwear, when wearing a bra top.
I also think "good for you" when I see people who are obviously struggling, for whatever reason, be it they are new to the sport, overweight, whatever. I just like people being somewhat covered up. G-d, I sound like a Puritan, and I am not. But, the shirtless male thing does get to me. And, not that many people look fabulous in lycra! If I hadn't found the 2 brands of shorts that I wear, I would look pretty bad, and I weigh 106 pounds.