First of all most of those Mii Costumes were themed after Streetpass, and they were hats, not costumes. Anyway, they were things Miis canonically wore, while these costumes weren't. So Mario, Luigi, Meta, etc, can't be put into the same basket as the others. And they were all already existing character anyway.
And you know...pairing Rool with Chrom makes it a pretty clear theme of "ballot favorites" which makes us think that this is a consolation prize...
It doesn't kill it 100%, but...it seems to communicate a pretty clear message.