I understand that 3rd-in-command doesn't necessarily mean 3rd most powerful, but Mundi still was a relatively powerful Master.
As opposed to the clones, Ki-Adi-Mundi along with all the other Jedi, was unaware of the betrayal till it was too late. Yes, Yoda was able to sense something was going on, but then again, it's Yoda. Ki-Adi at least managed to take out a few of the clones, before he was ultimately shot down, which is better than most other Jedi who were "out in the field of battle".
Ok, Cin Drallig was able to plough through many clones, before he was ultimately slain by Darth Vader, but those clones weren't already in his immediate vicinity like all the off-world Jedi. Those clones had to physically enter the Jedi Temple, while facing other Jedi, and eventually meeting Cin in battle on one of the many levels of the temple. Now this would give Drallig ample enough time to realise what was going on, therefore was able to formulate some kind of, albeit rushed, tactical plan, which in the end didn't pay off when facing Vader.
If Mundi was placed in Drallig's shoes and vice versa, it wouldn't have made much difference to the final outcome.