Wondering if Danny was a jerk before he got his powers because all Fun Danny wanted to do was relax and he was kinda selfish and irresponsible. Superhero Danny seemed to be the one to actually have a conscience. Apparently, the Danny we know and love comes from Superhero Danny but he didn't seem like a jerk when it was flashbacks of him being a little kid. What gives?
Hey there! I actually did a full analysis post on Fun!Danny and Super!Danny talking about why I think they acted the way they did. You can read the full thing here.
To sum it up, though, I think the two were the extreme versions of what Danny wanted for them, taking the parts of his personality that would best suit their roles. Neither of them was just Danny. They were two halves of Danny. So, Super!Danny took the altruism, responsibility, and other things that would suit his role as a full-time superhero, and Fun!Danny took the laziness, teenage rudeness, and other things that would fit a kid who wants to slack off and have fun all weekend.
So, no. Danny wasn’t a selfish jerk before getting his powers. His personality was the same, except less confident and more easily frightened. He has selfish moments like any kid, of course, but that doesn’t define his personality. It just helped define Fun!Danny’s.