It is mostly this. The fans of the SMT series are small but deeply devoted. Then Persona happened (well specifically Persona 3 and 4) and suddenly SMT becomes stunningly popular and they are outnumbered a thousand to one, Persona being to them a side game.
So there is a lot of fighting between the groups. The SMT people fighting for their little slice of the pie and Persona fighting for their respect. So in this case SMT fans just constantly rant and rave that "if a persona character gets in it will be a slap in the face" and all that.
The main SMT fanbase doesn't care about Persona and the main Persona Fanbase doesn't care about SMT... But the smaller subgroups and the trolls just make the conversations annoying...
I never knew this, actually. I've been a fan of the series for a fair number of years now and own five of the titles, and enjoy both SMT and Persona.
A series is allowed to have games of multiple styles within it's blanket. Do they also get butthurt over the Devil Summoner: Raidou Kuzunoha games for being realtime action, or Devil Survivor for being strategy RPGs?
In terms of characters for Smash, I would definitely like to see someone from Castlevania. Simon Belmont or Alucard would be the obvious choices, but Soma Cruz or Shanoa would also be cool (though entirely not going to happen).
Lyn from Fire Emblem: Sword of Flame is another impossible one to play especially since she is already an assist trophy, but would make a neat high speed attacker. Alternately Gilliam because I don't care what people say, he is the best General in Fire Emblem.
Also Rosalina.