I'd say pre-fight stuff is 70/80% acting/hyping the fight, maybe some fighters need to build some sort of hatred towards their opponents in order to perform.
Post fight stuff might have more reasons
1. pre fight stuff being taken too seriously
2. long term image building (if people hate you, they want to see you get smashed therefore you might fight more often and maybe even tougher opponents, just my 2cts here)
When there's that much money involved one can't be sure of anything
and as much as I kinda like this kind of hype, I fear it going towards some more "WWE"-ish kind of things, maybe Zuffa should go easy on that extra hype. I feel it's the easy route to success but at what cost?
And TBH, fans who take this thrash talking thing on 1st degree, well, they shouldn't, I'm working in marketing and I'm pretty sure, from my experience, that most of it is probably fake/acting (maybe some of em think at their reconversion, retirement plans and use it as an opportunity to train as actors?)