It's a tough issue, but I think I'm going to have to agree with Kupa in that we should buy the costume. Sure, it might send a message of contentment, but it would at least show that we like the character enough to buy the darn thing. I've never bought a single Mii costume because of how frivolous they are, and I'm still not sure I'll buy this one. But if the thing doesn't sell, that would be even worse because it would suggest that the character is not marketable, and it may have connotations of ungratefulness. If it sells well, then at least they might see that the character really does have the support base we often accuse them of ignoring. At the very least, if it sells, then that might encourage them to put that recent trademark to use and include him in more Donkey Kong games (and then perhaps Smash in the future). Heck, it's even the Rare design, so maybe we can get Rool back in that appearance for a double whammy. Not being hopeful here, but we should weigh our actions carefully and think about these things.
tl;dr - If it doesn't sell, Nintendo will probably either think the character lacks a following, or think we're a bunch of brats who won't even take a slice when denied the whole loaf. If it does sell, they may think we're being complicit, but at least they'll know the character has a legitimate following, and may consider him for the future. It's better than spiting them and discouraging them from using the character, even if it's for something as low-investment as a costume.