A sundress is much closer to a suit than it is to a T-shirt and jeans.

Considering modesty though, a woman in a sundress is equivalent to a man wearing a tank top. We don't get males in the office wearing tank tops or anything else that would expose the collarbone, armpits, shoulders, or shoulderblades. These things are normally considered inappropriate. We also don't normally see males wearing snug form-fitting shirts. It's not considered appropriate. Feel free to wear that stuff while you weed your lawn at home.

So that is two strikes against the sundress. It's kind of formal, yet no more modest than a tank top.

"We also don't normally see males wearing snug form-fitting shirts"

Heh you should really visit Shoreditch or Soho on a warm sunny day. ;-)

Often you can even take the same dress and dress it up to accompany someone in a suit or dress it down to accompany someone in jeans and a t-shirt.