Sakurai is a big Persona fan, and a massive SNK fan. Terry's review trailer started with a history lesson on SNK and the Neo-Geo
Sakurai speaks glowingly about most series he adds to Smash, because it would be very unprofessional and counter-productive to speak negatively about them. He also purposefully doesn't talk much about other games/series, because people always try to tie it to Smash. That gives the illusion Sakurai must be a big fan of these specific series, when really it's a function of Smash.
Not to say he isn't a fan of them, but dude spent what felt like five minutes just going over Tooty's route in the background of the Spiral Mountain stage. And told people to go play B-K on Xbox. If people didn't know any better, they'd say Sakurai is a huge fan of that series and that's why they got in. But we know why they got in. So we don't default to "Sakurai must be a fan".
Moreover, Sakurai speaks positively about most games he talks about. He's spoken positively about Sonic 06 and Cyberpunk shortly after its horrible launch. Not only that, but I feel like Sakurai has spoken the
least about Joker/Persona of the DLC characters, given it was the only one that predated the Sakurai presents format.
It's that we don't know why Joker and Terry
specifically were chosen, so people are just presuming it's bias. They're not only presuming it's bias, but completely overlooking where Sakurai said Nintendo chose the characters.
And tbf, I don't believe he wasn't at all involved in the selection process past feasibility, but the "Sakurai must be a fan" defense to explain inclusions usually arises mostly when there's no obvious better reason, not necessarily because it's the actual driving force. We just don't know what that force is.