The automatic solutions I heard of are Simplygon (Link) and Crytek’s CryEngine LOD Generator, didn’t know about Mixamo’s solution for this.
While I heard that those solutions work fairly well, I would be careful. Most likely an artists can do a better job, even though I wouldn’t be surprised if automatic methods get better than artists with time.
Getting Epic to do an own solution would be tedious (for them to come up with) and might not work as well. It sounds like a nice feature, but I’m not sure how well the end result would be.
Personally, from what I could gather, the best solution for now is Simplygon. It even has an engine integration, so it looks like what you are requesting.