Problem with clothes

I think the first thing you should do is to retopologize that clothing. Retopologizing basically means to reduce the polycount of the mesh either in quads or tris form. Let me explain better. Quads means 4 sides and tris means three sides, which is what your clothing has right now. If you want to be creating your own clothing for your sims, you would have to be doing that. Now that you know how to use blender, i will give you a link to a tool i use in blender, which you can download, to retopologize my sims clothings. Link1 and Link2. Make sure when you want to download both, you click on the green button that says clone or download, and in the drop down, click on download zip. Go to youtube and search for their names and there will be tutorials on how to use them. Once you are able to retopo your clothings, come back to the forum and it will make it easier for others to help you with the problems you have. Do this only if you feel like you will be continuously doing it for a long time. If you use marvelous designer, that is even more reason for you to approach the problem this way. I hope this helped.