I'd rather them do alt costumes right for three characters, then do them half baked for eight.
At least if a few are done in detail, it shows they are going to take the time to make the effort count, and that later efforts may be made for the others.
Yet in this case, I find the argument futile in general. I don't think Bowser Jr would even have had a roster spot if he was on the spot by himself I think the roster space is really for 'Koopa Kid' and that the main one is Jr, with all the other alts being just as much in focus. They each share the same role, and the realfighter is the clown car. The part of the series in Mario that it represents has to do with all the kids, and some of those games are ones that Jr was not even in.
I believe Sakurai struggled to add a highly requested character,JR, but did not want to give him an absolutely horrid move set (the paintbrush, from an already outdated game which would force JR to be a mixed dual clone). Instead, the character sot represents a huge assortment of Mario! It has the classic (well, older) games, bu having the Koopa kids, who also conversely represent a new era. It has JR who represents the in between time. Then it has the Clown Car, which is another Mario villain and a new part of the series represented, and it really shows of the goofy platform elements of the series in a way nothing else could.
Also, As for the work put into programming and voices (someone else can speak on the programming) but the voice work, as a voice over artist, I can assure you, unless there is some insane mode I cannot FATHOM (and I can fathom a lot) in Wii U, I will comment on. Each voice over artist was probably there for half a day, paid for a full, and only used for an hour (per voice) tops. Majority of the extra time would have been 'if something goes wrong' and the waiting game. The amount of sounds the characters make is excessively minimal, and very easy to replicate once in an actors repertoire.