there's a difference between serious things and things that you take seriously, for example:
exams are serious by nature so it's very hard to enjoy taking them
video games are fun by nature, you don't need to take them seriously, but you can if you want
there are characters who enjoy fighting, and there're character who fight because they need to, in my reading experience i can't recall seeing luffy having a fun fight against a villain, he always fights and gets stronger out of necessity, this is a first, it seems out of character, but it's not really that big of a deal to me.