It’s possible to take sports and other activities seriously and have fun too. I think many/most pro athletes enjoy what they do and/or have fun with it, while also taking it seriously, and they also enjoy having fun while not doing pro sports.
That is, they’re people, so they’re different. There are also some who are overly serious and/or miserable, and have no fun.
One can take their sport serious AND have fun.
One can take their sport serious and be miserable.
One can have fun in their sport and not take it serious.
One can be miserable in their sport and have fun.