I haven't seen any evidence that's true.

Magic Johnson and Larry Bird weren't. Kareem Abdul Jabbar wasn't. Hakeem Olajuwon wasn't. Brady and Manning aren't. Jerry Rice wasn't. Emmitt Smith wasn't. Tim Duncan isn't. Derek Jeter wasn't. Greg Maddux wasn't. Hank Aaron wasn't. Tony Gwynn wasn't. Ken Griffey Jr wasn't. Aaron Rodgers and Russell Wilson aren't.

I'd argue that the radical majority of elite athletes are not.

Even Tiger Woods, who is famous for being chilly in public, isn't known to be a big ahole despite whatever his personal life failings are/were.

I think that, while elite athletes all have a competitive streak, there's a lot of natural variation in personality as well. Larry Bird may not have been an asshole off the court, but if you were guarding him it was a different story.

The other thing to note is that, as long as you avoid doing something stupid like throwing people through plate glass windows, it's easy for PR to do a lot of magic. Most people never realized what MJ was really like while he was still playing. Brett Favre was kind of a dick, but it took most of his career until that was found out. Ultimately we don't know what these people are really like. Who knows what lies behind Tim Duncan's placid exterior?

Wayne Gretzky wasn't!

(Come on!)

Federer seems like a decent enough guy...