Let's give a hypothetical. Say you're wearing a bra or something underneath a shirt that doesn't exactly fit 100%, or a pair of shorts that clip into your shirt.
Frustrating, right?
It should be possible, to set per-attachment adjustments to the rigging of fitted mesh on an avatar. The bust area on a shirt can be enlarged to accomodate for a bra, or the shirt's waist area can be expanded to move it away from the waist a bit to mitigate clipping.