I wouldn't worry too much. Even if initial numbers aren't huge, legs will carry it a long way. It appeals to the same crowd as Animal Crossing, a game which has been out for around 14 months and is approaching 40 million sales.
But these are huge initial numbers indeed. You have to compare game sales to something comparable. Comparing the biggest multiplatfrom horror franchise ever in this case isn't meaningful.
The most meaningful comparison would be to compare to the original Miitopia release.
And this port sold 36% better than its original.
When a HD port sells better than its original then we can talk about success because the original was successful enough to make them consider this port and it is even more successful.
Of course this does not apply to Wii U ports as in that case the platform availability crippled the sales often.
It is also important to note that we are talking about boxed sales only and for 3DS 4 years ago digital purchases were not so widespread than nowadays on Switch. The game was no 1 on eshop best seller list.
