Honestly, did he have a point being in this game other than a lesser-occuring annoyance compared to Kamek?
We see him three times; 1-1, the intersection at sea, and finally at 6-2, where he met his quick demise.
Apparently he had a desire for rare stickers that rivaled Mario's quest for the royal stickers. Also, he had the power to paperize, as seen in 1-1.
However, not only do we no longer see his powers after that stage, but he started to grow a dependency for his Koopa Clown Car, covering himself with a shield and throwing what I presume to be the spiked ball reserves from inside the Car.
And just like that in 6-2, he vanished without a trace. No further mention from Kamek, his mute father, or any toad that cowered from him in the first level.
He appeared out of nowhere, whining about stickers, as well as his dad getting mad, but that never showed whatsoever in the game.
It seems to me as though they originally planned to use him as a constant rival of sticker power - much like Jr. Troopa and how he tried to grow stronger to beat Mario - however, during the development, they tried to scrap that idea and sweep Bowser Jr. under the rug nonchalantly. Another hypothesis is that they wanted to create a foil to Kersti - who by comparison is calm and not greedy, for she only wants to complete her mission - so that we would feel for her upon her sacrifice in 6-3. (Though if that's true, I believe that attempt failed...)