As Sim gain weight, the quality of her clothes deteriorates

Hi. Your problem lies in the polycount. 55 thousands of tris for this garment is too much.
Higher wireframe density = worse  result of the weight transfer
When you transfer weights from low poly to high poly meshes, they do not transfer quite like you'd want them to.


Also, when transferring a uv_1 map, use Nearest face interpolated instead of the default one


Ideally you would want to retopologize the mesh. If not, at least go back in Marvelous designer, quadrangulate the mesh and raise Particle distance at least to 20.