Mugray struggled to return Isner's serve on slow(ish) court of USO, he was struggling to break him and spent like 4 hours outlasting him physically because he couldn't do shit against his serve. Mugray is an overrated returner, sure he can return Nadal's serve like nothing (who can't), but the real test is against a BIG server and that is where Djoker is better.
it certainly wasn't the return that was the problem in that match it was the fact Isner was playing brilliant from the baseline in sets three and four and outhitting Murray, but cause he's an 'all serve clown' you won't admit that.
BTW Djoker needed 5 sets to beat him on an indoor clay court in Davis Cup.
Murray's eaten Karlovic's serve up on a few occasions, whereas Djokovic hasn't. He hasn't faced any big servers this year and his return stats are skewed by facing clown serves like Nadal's.