Isabelle is not a good example. She may not be as big as she is in Japan, but she still carries a lot of popularity in the west and even ignoring that Animal Crossing is a highly successful series that does well across the globe. She nor her series are anywhere near Mortal Kombat's struggles in Japan and they're nowhere near comparable.
And really, that's the big thing with Mortal Kombat. Unlike most of these examples (only Dark Samus is similar, and she's an Echo so she got in by being easier to add) it struggles in Japan whereas the others at least have some notable presence outside of their market. Dragon Quest for instance has sold a million at least for some of its entries including the recent Dragon Quest 11, Banjo actually did well in Japan and the original sold over 400K there, King of Fighters was a notable feature of arcades even if it wasn't quite to the level of Street Fighter, and so on and so forth. Mortal Kombat by comparison struggles to sell much in Japan and sometimes doesn't even get released there, with the most recent entry (XI) skipping it entirely. So Japan's lack of familiarity with it is a hurdle that it'd have to jump if it has any hope of getting a character in, and it should be kept in mind when talking about it.