People don't realize how amazing the Flash Gordon line is. Stark is a grown man who sees the character as a campy childish caricature of a hero who dwells in space which is in a way what Star-Lord is but he doesn't mean it as a compliment. Quill on the other hand grew up with movies like that, didn't get a chance to watch them as an adult and say wow this is ridiculous. He just remembers Earths culture from the perspective of a child so it's all super cool. Flash Gordon was most likely a movie he remembers and more than likely loves to see himself that way. So for him to hear that is probably the coolest thing he's ever heard anyone say about him, when it's really an insult. One of the best most underrated comedic moments in the MCU.