It's a release many people has been waiting for many years, I think it was expected to see good results. Unfortunately, given current Wii U's state (i.e. very, very low installed base), we don't know how much "good" is being in the top places of eShop charts.
Well, it had its fair share of doubters -- people who insisted there was a small but vocal fanbase... and little more than that. Even if the absolute numbers aren't large, soaring to the top of the charts shows that it can outsell many of the games Nintendo has considered "safe bets" for a while.