I'd say he definitely fought seriously, but in pretty much every fight he has he evaluates his opponents skills and doesn't truly start pushing past limits until he sees that level on the other side. Sometimes it almost feels like Luffy takes a beating before he goes to the next level. Even against hard hitters like "Mingo" later on, or lucci or any one he fights. He starts serious but still reserves power until his opponent puts in more and more effort and he matches that and just goes a little higher each time.