Why'd someone that's a heel not want to show themselves as that in UFC when that helps selling PPV for fights? That'd be a good question that'd challenge this notion imo.
But it's also possible for someone to be a heel but to be extremely aware of burdens it can give, criticism and general support from fans, which while most think it doesn't matter for fighters, it might do a little, I imagine. So maybe a heel manages through self control hold it cuz of fear of self image and what is socially accepted as nice.
I think when someone has more like a heel personality in fighting, but have the need to protect it stronger, it may break in pressure situations when a big rival threatening you emerges.
Ignoring JJ's outside of the octagon saddening situations, looking only at comments on social media related to the fight, press conferences and such, it seems he had more of a nice guy vibe, or a seemingly average dude that turned to killer mode in the octagon, didn't trash talk or provoke a lot, but the heel would show up sometimes like not holding Machida when he choked him ans needing Greg Jackson to tell him to check on him to get some fans. The extreme violence in some fights, which while it is violent, sometimes it seemed like he was prolonging some fights like vs Shogun and getting in as many shots as possible... when DC came up, maybe this heel came full force.
Idk tho... I think examples ppl could pick of Valentina are her training regimen saying no water allowed, that they must het used to suffering to get tougher, or her dismissive comment after her loss to Alexa Grasso 1, while in the outside showing good sportsmanship, also complementing by saying she was in control of the fight and had a distraction...