The MATE team is small and it would unfortunately be a lot of work to keep a fork like this maintained. (That said, I only speak about the MATE team, not for them or as part of them.) I don't think they'd say no to someone who creates and offers to maintain a fork, however
Another option you can try is installing the
gtk3-nocsd package. This is a "A hack to disable gtk+ 3 client side decoration". I personally haven't tried it, though.