I never liked him to be honest, I prefer the Koopalings from Mario games before Sunshine. Maybe it's just his annoying voice in Sunshine or something, but I just didn't ever like him because of that really.
I really don't like Bowser Jr. I found his voice annoying, it lacks the creativity of the Koopalings, and I think he was just designed to be Bowser's sidekick.
I like him more than the Koopalings. Their big issue is, putting them in spin-offs, you need all of them playable, wiping out seven spots. With Bowser Jr., he can actually replace Bowser. I mean, in the TV shows, I liked the Koopalings. But, I prefer Bowser Jr. now.