Does github need a cartoonish cat with 5 octopus-like legs to be its logo? Of course not, but it makes it memorable and funny. And besides, anime is extremely mainstream these days.
Then you must be old. Even in western countries Spy x Family (which the character is from) has sold millions of copies, while most people read mangas online and won't be counted. In the country I am from I frequently see people wearing merch of it, mostly because Uniqlo has had a successful line of it. And that is just one manga/anime out of hundreds of popular ones.
Using anime characters is similar to boomer nerds referencing Marvel/DC comics , Star Wars etc.