Yup. As silly as it sounds, people use it as if it were a real word. If you hadn't guessed, it's a cross between "math" and "athlete." It's used to describe people who compete in math competitions (such as MathCounts), possibly as a way to make them feel better about being such big dorks, or maybe to pretend that people will ever take math competitions as seriously as sports.

I was a teenage mathlete.

You know, it's true. I really was a teenage mathlete, and I competed in MathCounts at the national level and other things locally. I think I can pretty authoritatively say that nearly everyone involved was a big dork. I still am one (and I'm proud to be). Also, hardly anyone who isn't involved is aware that mathletes exist. Whether they'd be better off knowing is up to debate. It's up for debate if their lives would be better if they knew or made a big deal about it. As the captain of the math team, I sure wasn't treated like the star quarterback, but I had my share of fun.