Muguruza is not like that. Tournaments she is really playing well in, she keeps playing well. Such tournaments are the 2 she won, U.S Open 2016 (which she would have won without question but running into Kvitová's A+ game, and still wins if she pulls out that 2nd set and goes on to win), a few others. If she plays well enough to beat Halep somehow she isn't going to throw in a total stinker in the final and lose to someone like Kenin.
I agree she is still a better match up for Kenin than Halep though. Halep is essentialy just a way better version of Kenin, and won't choke like Barty today even if she winds up playing badly for her she still would beat Kenin.