Mesh warping on non-skinny Sims

Hi. There are two reasons:
- the dress does not morph properly because you vertex painted it with the wrong color. Dresses are painted with two, the skirt area has 3FFF00, the rest has 00FF00. In your case, the whole thing is painted with 3FFF00, so that is why it behaves wrong
- your blend file contains only one meshgroup, whereas the package has 3. Only one is being replaced while the other two stay. You need to check how many meshgroups items have before importing the blend

Unrelated to the problem issues:
- when you make a dress, you need to start the package from a dress of the same length and use it as a reference. If there is nothing quite like this among maxis items, there is always a reason for that, so do not do what you have in mind, at least not as a beginner;
- If the dress receives all the data from a swimsuit, it will move and morph as a swimsuit instead of the dress;
- In Md model in quads, not tris, and do not lower the particle distance. Right now the item's polycount is 3 times higher than that of the most complex maxis meshes;