Dress Making 2 Quick Question

Hi.
1). First, you should either combine the top body part with the bottom one or just use a Base Game lingerie set as a whole body;
2). Overlapping of the outfit's UVs with the body's UVs is okay only if that part of the body is completely covered by clothing ( which is not the case for how you put your UVs here). In The Sims 4 there is only one UV-space (for CAS items). There is only one diffuse texture map that is split into areas for different CAS parts. So when you put the dress' UVs over the UVs of the body part that is not hidden by it, you end up with the texture for a dress on the body.

How I suggest you to go about it:
1). The topology ( think of it as "a way in which vertices your mesh is made of are placed and connected )of your mesh is bad which will result in:
- higher polycount (the number of triangles (polygons)) that will increase the loading time of the dress both in s4s and the game;
- worse quality of the diffuse map ( if you plan on baking it );
- the mesh deforming in a less smooth way in the process of moving and adjusting the body parameters;
- harder time selecting border edges;
Ideally, you would want to retopologize it, but since you're new to this, I suggest to just change the mesh type and raise the density. To do it, go back to Marvelous Designer and change Mesh type to Quad ( 1 ). You can set it as default in Settings - User settings - 3D - Mesh - Quad ( 2 ). Then adjust Particle distance ( 3 ) in a way that the mesh would still look smooth but not very dense. Run the simulation. Export the mesh as Unweld.
2). Start a new package by cloning this lingerie set ( 4 ). Export the blend and open it. Import the obj file of your dress.
3). Select the body. Switch to Edit mode ( TAB ). Disable limit selection to visible ( 5 ). Switch to Vertex Select ( 6 ). Switch to orthogonal ( 5 key ) front ( 1 key ) view. Holding Ctrl, lasso-select the parts of the body that are completely hidden by the dress. Rotate the mesh and add more vertices to the selection or remove unwanted ( holding Shift-Ctrl ). Once done, press Delete key and choose Faces ( 7 ). Make sure to leave one row of faces at the very bottom ( 8 ). Otherwise, there will be holes in the process of walking.
4). Once done, press A to select everything. Choose a background image ( 9 ). Shift-select the dress. Switch to Edit mode. Press A and choose the same background image. Enable Draw other Objects ( 10 ). Now you can see the wireframe of the body.
5). in UV Editor press S to scale, Y to scale vertically, .5 to shrink the UVs in half. Using S ( scale), G ( move ), R ( rotate ), place your UVs in the vacant place of the part of the UV map that is dedicated to tops. You can move and rotate UV islands independently but not scale! Everything has to be uniform in size.