You can scroll back to 2018 posting when they first announced flights I think out of ORD and see it was quite popular. I haven't ridden on EVA's Hello Kitty flights, just on the regular Biz service a few years ago and service was great. The SF Bay Area has a very large Asian community so plenty of people going to-from TPE and onwards.
Hello Kitty was just always around for me (it started ~1974 - I was born in '77) and I got plenty of Hello Kitty stuff as presents from relatives growing up. So for me Hello Kitty is more nostalgic fun. I ate Hello Kitty dim sum in Hong Kong in 2015 and my sister bought me a foot-tall Hello Kitty stuffed toy. (You can see pics in my
trip report) This year I rode on a
Hello Kitty branded airport train from KIX to Kyoto. I just took the first train that arrived, so it was a fun surprise to see it was the Hello Kitty one. It wasn't extensive like EVA's branding - you can see in the link how its much more subtle than the
Shinkansen - now that is a pink explosion! Yikes!
United did a Spider-man branded thing for a few months to coincide with the movie release -
New Spider-man Themed Safety Video & Amenity Kit - so future branding from other popular characters may happen in the future on other airlines. It's marketing, and if someone pays them enough money, it'll probably happen.