hi! ever since i've started using cc, i've noticed a problem with many skirts and dresses - they seem to morph weirdly on plus sized sims. after learning to make my own cc, i finally found the culprit - it's the incorrectly applied vertex paint.
in this tutorial i will explain how to fix this to finally let your plus sized sims have cool clothes too! this is aimed at people who don't make cc and have little to no prior knowledge of blender. if you're making your own cc, you can skip most of the steps and just transfer vertex paint together with uv_1 and weights (that's how it's supposed to be done, anyway). if you are a cc clothing creator please please always check how your cc looks on different body presets, this is so important!
to not point fingers at any creators, i have made the example from my own dress. you can download the correct version with proper vertex paint here: https://boosty.to/moorshum/posts/7e7b8fb9-e915-4f37-aedb-0bcc37cf5f9f
 before we start, you will need two free programs: blender and sims 4 studio. there are plenty of tutorials on how to install and use them so i won't focus too much on that and will only explain the steps that you need to do to fix the specific issue i mentioned earlier.
 open sims 4 studio. you will need to enter your creator name to use it - just enter anything. press the "my projects" button - it should open the file manager. you'll have to find the cc you want to fix in your mods folder. when you open it, you will see something like this:

go to the "meshes" tab and export the LOD0 mesh somewhere. just make sure you name it in a way that is easy to remember!
now we need to also find a reference mesh to transfer the vertex paint from. go back to the main menu. select the "create CAS standalone". in the catalogue, find a clothing item that is similar in length to yours or longer (it has to be a dress or a skirt, same as your og item) and hit "next". save the package somewhere (you won't need this one). export the LOD0 mesh the same way you did before.

now open the original clothing mesh in blender. you'll want to add the reference too, at the top menu click on file -> append -> navigate to your ref mesh -> object. you want to select all the parts that say s4studio_mesh_[number]. if there is only one, feel free to skip to the next part. otherwise, in the menu on the right shift+click all of the appended parts and press ctrl+j to join them. rename it to "ref" or something like that for convenience. make sure the reference is visible (the eye-looking icon on the right would be open)

delete the appended rig and bone_bone_shape, we won't need them.

choose your original mesh and click on the wrench icon. click "add modifier" -> edit -> data transfer. source should be your ref. check "face corner data", in mapping choose "nearest face interpolated". click the arrow at the top right and hit apply.

we're pretty much done! just delete the ref and save your mesh, ideally name the file differently so that you have a backup in case something goes wrong. import the mesh into the cc package in sims 4 studio and save. launch the game and test with different body presets.
if you don't want the issue to happen when zooming out in live mode, you'd have to also do the data transfer to LODs 1-3. yes, it would be insanely tedious if you want to fix a lot of cc. that's why i would really appreciate if you share this post and spread awareness about the issue, since even very experienced and popular cc creators have it, it's VERY common. thank you if you made it this far! feel free to ask if anything is unclear!
little note: with some custom body presets, the clothing will still indent a little between the legs on bigger sims. i'm not exactly sure why that happens and how to fix it so just be aware of that!