But they actually have been creative with the Koopalings. Every game they've been in their fights go differently. The big problem with the Koopalings is that it's too easy to hit them 3 times before they really get a chance to attack, which is a problem the Broodals also had. The Broodal fights themselves were exactly the kind of thing a Koopaling fight would be. I like the Koopalings, too, and at least if they went with the Koopalings Nintendo maybe would have given use 7/8 unique bosses instead of 4. Hell, Bowser Jr should have been the ring bearer. Wendy the flower girl. I'd love to see the Koopalings in 3D.
Bowser Jr's first appearance in Sunshine was really lame, but every game he's been in since then has solidified him as a worthy addition to the Mario universe. I love how they've characterized him as Bowser's general, like a more competent Bowser who gets it done but still bows to his dad. I really like that they made the junior clown car his gimmick. That was a really good idea. Bowser Jr's boss fights in NSMBWii and NSMBU were great.
When were the creative with Koopalings? I can excuse SMB3 because of its era -- but some of those Koopalings were identical in mechanics. SMW, same deal... I can't think of any title where each of the Koopalings individually has some interesting boss mechanic.
Bowser Jr. has gotten so much care over time. I wish they could do that with each of the Koopalings in some new title.