That's the thing, Mario is a widely known IP. Everyone and their moms knows Mario, he is globally recognizable...even in places where Nintendo is historically weak (China, Europe). However, the people who are willing to spend on the IP, and the degree of spending, is very likely very much smaller than Fate.
Nintendo IPs in general are not well-leveraged, not well-exposed and not well-exploited compared to anything by Disney...or even Ghibli. and Fate This is where Nintendo falls flat, it has an icon that is almost as iconic as Mickey Mouse, but has failed to keep in the non-gaming populace's mind to induce them to spend money on the IP. This is why Nintendo is shifting its strategy, after the abject failures in video game the last generation, to leverage its IP more in other non-video game areas. However, that will take time to ramp up...and it has not quite reached that phase yet.
Also, there is no way Fate doesn't have mainstream appeal at this stage (post FGO), especially in Japan. Going by imperfect social media metrics, If Splatoon is the new hotness (with 1 million twitter followers) in Japan for Nintendo, then FGO alone is much more popular than Splatoon (1.22 million twitter followers). I bet Fate also has more mind-share among the foreign weeb population compared to Mario, Zelda or DQ (Pokemon still wins though).