1:The mesh shape is very distorted, or otherwise doesn't look realistic. Custom content doesn't have to be perfectly realistic, but it should have a plausible shape, and obey most of the laws of physics.
2:The mesh shape is a little distorted, or needs some work to make it look realistic. Custom content doesn't have to be perfectly realistic, but it should be close: meshes should normally respect gravity, sims' proportions should be believable, and so on.
3:The mesh shape isn't perfectly realistic, or has some small distortions or other problems, but it's pretty good overall.
4:Your mesh is realistic, hangs naturally, and is generally very good!
5:Your mesh is fantastic: the shape, hang, proportions and other elements are excellent. Nice one!