One extra mutation per level is a considerable boost but not really OP. The teratomorph substitution mechanic may be troublesome, however.
The Limitless mutation is a huge (and frankly pointless) sink - edit or at best a method for acquiring a later ability early. The Skill Boost mutation is a massive sink that is objectively inferior to a most skill mutations or the ability score increase mutations. The Mimic acf is... Kind of a boring class feature, all said.