Mesh is 97k!! Can I reduce it without destroying the look?

simmerish20's method is what I use. Hopefully, the dress has decent topology. If it's the crazy topology that typically comes out of Marvelous Designer dissolving edge loops won't work. The whole thing will need to be retopologized. Otherwise, do this (and some of this simmerish20 already mentioned):

1. Select your mesh and tab into edit mode. 
2. Press A to select everything. In the UV editor press A to select everything. Both the mesh and the UV islands should turn orange.
3. In the UV editor, select UV > seams from islands. The seams will turn red on your mesh. Hopefully, the UV islands are clean and not chopped up. If they are, that will mean more work to fix the UV islands. 
4. Enter edge select mode. Select one edge marked red. Doesn't matter which one. 
5. Use select > select similar > seams. Or you can use the Shift + G shortcut and then select "seams."
6. With all your seams selected, mark them sharp. They normally will turn blue but if they are red you won't see the blue so don't stress about that. 
7. In edit mode, press A to select everything then merge doubles. In versions below 2.80 you press W > remove doubles. In version above 2.80 it's either M or Alt + M > by distance. 
8. With your mesh still in edit mode and still selected. Press T to open the side panel if you're using a version of Blender below 2.80. If not, skip this step. 
9. Press Alt + J to turn the tris to quads. Note that it doesn't always work in some areas and will require manual fixing. 
10. In older versions of Blender, put a checkmark next to "compare UVs" in the side panel you just opened. In newer versions of Blender, you'll see a little window in the lower left corner. Expand that and put a checkmark next to "compare UVs." Sometimes turning tris to quads messes up the UV layout and compare UVs fixes that. 
11. As I stated, turning tris to quads doesn't always work perfectly and you may need to add edges or remove edges Blender didn't catch. You can select two vertices and press J to add a new edge. If you have one face that is still in tris and needs to be a quad, select the entire face and press F, or you can select the unwanted edge splitting the face and press X > dissolve edges. 
12. At this point, you should have a mesh that has doubles removed, seams marked, and seams marked sharp, and your tris should be in quads. 
13. Hold Alt + Shift and select edge loops you want to get rid of and press X > dissolve edges. Focus on areas that are really unnecessarily dense. You may find that removing an edge loop changes the shape of the mesh. If it does, don't remove that edge loop. For small things, like the buttons, for example, if they have a ton of polygons get rid of a lot of them. Those are areas that won't be seen much, anyway. 
14. Once you've removed all of the edge loops you want to get rid of, use the edge split modifier to split the edges again, and save your file. 
15. I go through this process reduce the polycount for every LOD. Remove the doubles, dissolve edge loops, use the edge split modifier and save my file. For example, I take the LOD 0 and remove edge loops if needed, do what I described above and save the file. Then I take that same mesh and go through the whole process for my LOD 1. Then I repeat the process again for LOD 2, and so on. Each LOD has fewer edge loops than the previous LOD. 

As I stated though, some meshes are in such horrendous condition that it's not worth it to fix. If the topology is the tris that MD produces you can't use the dissolve edges method. You'll have to use the decimate modifier or retopologize the entire mesh. And the UV islands could have weird splits in them/are choppy. This will require more work to fix. Here's an example of choppy UVs:



Here's an example of horrible MD topology:

I know, right?! I don't think I'm good enough to fix this. If you plan on doing it, do share because I'd love to have it hahah

I guess I'll just try to find something similar, this is gonna turn out to be a chore and probably not worth it. Thanks guys!

I started to work on it, lithops, but after an hour I gave up in frustration. It's going to take days to properly fix. At least it would take me days to fix it. The only quick option that I can think of is to use the decimate modifier. The mesh's topology is already halfway wrecked so you might as well wreck it entirely, lol. It looks like the creator used the remesh function in Marvelous Designer but then said "screw it" halfway through the process and left it as is. It's too bad because the dress really is beautiful. The creator's other stuff is beautiful, too, but who knows what kind of condition they're in. 

eunosims.tistory.com/entry/sims4cc-everyday-Hanbok
Here

Do you mean that you can't download the file? Or open it in S4S? I can try and see if it exports for me

These are probably noob suggestions you already know about, but just in case...

I'm not sure it would fix *this* project as it sounds like there's a ton of extra geometry and problems, but I'd suggest baking out an AO and DIFF at least of the high poly dress, then decimating the heck out of it. Once you've got your dress arranged for import to CAS and bake out your textures, replace the AO and DIFF you just baked with the high-res ones you baked earlier before decimating by just cutting and pasting layers in Photoshop (or your 2d editor of choice) and arranging them over the top of your bake for CAS. If you change your primary textures to white, you can probably use this new patchwork AO to create a normals map that will help restore a lot of the look of the original shape.

Also, if you can manage to select edge loops you want to maintain (because it sounds like the geometry is a mess), you can reinforce those edges by creating a second edge loop as close to the first as possible. When you use the decimate modifier, these double edges should preserve the shape of that particular edge loop.

Like I said, this is probably all old news, but just in case I thought I'd mention it. Good luck!