Well the good news is Unreal 4 is modular so one is no longer limited to a single solution and one can use anything they wish that gets the job done. This change of how things goes from here to over there opens a boat load of options so sure Mixamo is an option based on the must have requirements of royalty free licensing.
Whats cool about Mixamo is it’s focus on video game character models, even though it uses the Mister potato head approach, and the main selling point I think is in expatiation of quality for the price.
Not really all that keen on subscription based services, it all adds up fast, but for the price the Indie package is on my shopping list just to add another option to our usable resource catalog.
MakeHuman in my opinion is not mature enough “yet” to be usable with out additional work. On the other hand it’s CC-Zero so if your into making a game on the cheap it’s probably the best option that fits this need.
As an opinion though the power tool as to parametric character design is Genesis which comes as part of the free Daz Studio download.
The downside is Daz Studio is a very complex application designed to meet a lot of different needs with a fair size learning curve.
Overall though for 99 bucks for quality that’s one hell of a deal.