I have 2 2400 cp plus Muks, one I used as an attacker, the other a defender.
For me, defense is pointless, and him being a raid boss certainly ruined the cool factor (mine were at these cps before raids.) I wouldn't invest in a defender.
However, on offense, Muk is very solid as a generalist attacker. He is extremely bulky, even against zh Snorlax and Blissey. I usually used him against both of them and as my clean up against Blissey if I needed it. He does neutral damage to most defenders. I would invest here. You then have very viable Pokemon you can have fun with.