Regular Show finale kind of sucked.
It was almost as unfocused as the Adventure Time finale. Mordecai and Rigby practically weren't main characters anymore. It wasn't really their story. It was a story of Pop's evil twin brother appearing out of nowhere trying to destroy the world and fighting everyone (except, you know, the dozens of the memorable recurring characters the show had amassed over the years, most of whom didn't even appear).
Also the internet helps him destroy the world for some reason.
Also the creators pat themselves on the back with endless meta jokes and 4th wall breaks, none of which really amount to anything meaningful or compelling.
Also the fact that Regular Show was distinctly NOT one of the "deepest lore" shows yet they tried hamfistedly to fit it into that mold in the final season because that was trending at the time, with the whole dumb space supblot.
Ugh.

I guess the only reasons people don't shit on it actively are:
- Mordecai got laid at the end
- there were no explicit SJW themes
I guess that's all it takes to please an average /co/omer, huh.