Hardcore fans don't like the idea but casual fans do and I'm pretty sure there are more casual fans.
I think Joker would have made it in even if that wasn't a thing because Sakurai wanted it. I guarantee it didn't happen solely because of a Koei-Tecmo spin-off.
Unrelated, Tracer in Smash would be great. Way more interesting than the same three-four characters everyone keeps saying.
Are there any characters in Smash that don't have a game in a Nintendo system? Undoubtedly Joker was tied into the Switch release of Persona 5 S. Only characters I can think of is Cloud and Snake although there have been multiple FF and MGS games on Nintendo systems. Those two are also iconic gaming characters nonetheless unlike Joker.