How do you call the people who love baseball?
Do you usually use "baseball mania" or "baseball lover"?
Well, Frank Thomas has already answered in a comment but
According to the Cambridge Dictionary
someone who admires and supports a person, sport, sports team, etc
He's an avid football fan.
Just replace "football" by "baseball" in the example.
There is even a movie starring Robert DeNiro about a baseball lover titled The Fan.
"Baseball fan" would be the obvious expression, but there is nothing wrong with "baseball lover".
The noun would be "baseball maniac". I wouldn't use that. It sounds like someone who is actually insane