Mainstream:

the ideas, attitudes, or activities that are regarded as normal or conventional; the dominant trend in opinion, fashion, or the arts.

Not really.

According to the definition of mainstream, Persona is nowhere near close. If it was mainstream, then it would be considered the "go to" for what people consider to be the most popular RPG that has shaped the industry. This is not the case. The only RPGs that have shaped the industry are Final Fantasy, Dragon Quest, and Pokemon. As every single RPG in existence has taken ideas and concepts from those games. Persona is not considered "the" premier RPG that other RPGs take after or have learned from. It hasn't even sold that well either.

Fire Emblem Three Houses has sold three million copies within one year while it took Persona 5 multiple years to accomplish the same thing despite being on a console with a larger install base (PS4's 100M to Switch's 68M), and in a more popular genre (turn-based is more popular than tactical). Yet Three Houses outdid it in sales speed and popularity.

There's a reason why they needed to announce Joker at the Game Awards while Hero only needed to be announced at regular Direct. The amount of people that knows DQ are way higher so it didn't need a big announcement to catch people's attention.

Is Persona popular? Yes. Does it have a vocal fanbase? Yes. But is it mainstream? No, not really. Not until we see dozens of other RPGs take after it like Final Fantasy has done.