Many of the vendors in SL are the same folks making things for DAZ/Poser. They use the same tools, Marvelous, Maya, etc and the basis for a working model is based on Daz models. While good design requires trying to keep polys as low as possible, the game has implemented a lot of tools in order to accommodate 'bad design' of high poly models, like distance rendering control, etc.
Given all that, with clothing, you will be working with an OBJ file or a DAE, so you have to rig it.