Mixamo should have an option to export an Unreal skeleton, which means it will be compatible with ART’s import motion tool:
You’re on the right path to skinning and using a custom mesh, but it takes a lot more work to get good looking results The defaults will never look good. There are tons of maya skinning tutorials out on the web. It’s a very technical, artistic, and sometimes, difficult process. It takes years to get good at creating good deformations. At least, it did for me.
There is no tool in the world that I’ve ever seen do amazing automatic skinning. The ones that do often require an input of tons of other shapes to compare against, like 100 facial shapes to then figure out skinning from.