UI Overlay mods are tricky to upload on Curseforge without getting instantly rejected and the fix is not obvious. I had contacted Curseforge support awhile back and this is a working method they provided me with:
You have to remove the [MODNAME].mod file from the Meta folder in your project WHILE Modkit is running (very important!) and upload the mod right then and there.
Once it is uploaded you have to put the .mod file back into the Meta folder. If you do not, the mod will not show up in Modkit the next time you load it.
This is a strange quirk that only affects UI Overlay mods and I suspect a lot of people haven’t gotten past it as there are way more UI mods on Nexus than Curseforge. Hopefully this will clear up a lot!