KarlaCommunity Moderator, iVillage.com
I think it whether wearing a sports bra alone looks good depends on the bra, not the person wearing it...
1. Is it one that is made to be worn uncovered? and2. Is the fit and support sufficient for the woman wearing it?
I think as long as a woman is wearing a good fitting, sufficiently supportive bra that is designed to be able to be worn alone, it's fine to wear it.
Actually, that's pretty much the test of whether any item of clothing should be worn...more generally:
1. Is it appropriate for the circumstances? and2. Does it fit properly?