It's not always the case that well constructed and fun games achieve deserved success wherever they go, and Mario Golf: World Tour is a case in point. In the UK charts — which only last year frequently had multiple 3DS titles populating the top 40 — it's endured a tough second week, in which it's also the only Nintendo title in the list.
After making a modest all-format début in 22nd place, the second week on the UK market has seen Camelot's title fall way down to 37th position; on current momentum it'll disappear out of the top 40 in its 3rd week. There are some multi-format gems that have had a tough critical reception doing fairly well, however, such as The Amazing Spider-Man 2 in 3rd and, most outrageously, Aliens: Colonial Marines which is still up there in 27th place.
The UK is just one market in Europe, of course, while we'll find out in mid-June how it's performed in the U.S., too. It seems that this title simply hasn't captured the imagination in one country, unfortunately.