If the BJJ guy jumped on his back, Thor could just throw himself onto his back on the ground. I think 200kg landing on him might make him lose his grip.
I get that skill helps a lot, but I just don't think it can overcome that big a strength discrepancy. Or at least not 100% of the time. Of course, BJJ is flavour of the month now and everyone who does it thinks it turns them into some sort of invincible jedi. This despite the fact that more fights are won in the UFC by knockout than submission.
UFC knockouts & submissions 2014-2020 | Statista