Welcome!
It shouldn't ever be easier to beat good players than bad players. If it is, you are either playing worse against the bad players, or you are mistaken as to what makes players good or bad. You make money when people call 7-10x raises with trash, even if you have short-term frustration.
It's basically impossible to put MP on KK since he flatted the PF raise, obv 66 is a risk but that's one combo. But yeah we do have to think about what he can pay off our jam with. Sometimes it makes sense to just call, even if you think you are good 95+% of the time, if there aren't hands that we beat that pay off a raise. One easy example is 4 hearts on board, we hold the king, someone bets. If they aren't super tight, we win the hand quite a lot of the time. But if we raise, most players will fold everything but the ace unless we are a maniac, so it usually just makes sense to "call expecting to win."
In this hand, it would help to know what the turn blank was. Would it be possible that he turned 2 pair (could if it was a 5 or 7, could not if it was a 2, for example)? If so then he could have rivered a lower boat. Did the turn bring a flush draw that didn't include the 6? If so he could have a 6 with a flush draw, and then made trips. Would this player pay off a jam with trips?