I wouldn't. It's not a terrible language to work with on the job, but I don't think I've gotten anything out of it in the same way that I did from learning OCaml or Lisp.
Ditto, there's not a whole lot of innovation of ideas in MUMPS at all. It's just a random programming language that's good enough to get things done with and the database that's next to it is nicer than most and supports both NoSQL and SQL. Spend the time on learning Python, JavaScript and SQL I'd say :P