I don't think Persona is really the kind of franchise that would cause a noticeable spike in sales. I think Joker may have been an effort to strengthen the bonds with Atlus and get future Persona games on Nintendo consoles. Around the time Joker was released, Sony randomly made a comment about how they're very proud to have Persona 5 as an exclusive. They already lost Undertale being an exclusive, and the odds of Toby Fox coming back are slim after how much that's happened with him and Nintendo this year.
Partly that, yes - but it's also partly because Persona 5 has one of the most unique styles out there. If I may channel my inner connoisseur (think Rarity, my personal favorite Main 6 Pony), lets look at the menu screen:
Does this look like something that would fit Smash's style at a glance? Before Joker was revealed, I bet a lot would say no. The two games don't have matching color contrasts, themes / aesthetics etc. But there's the catch: Joker's trailer was in P5's very distinctive style, and only showed the Smash letter well into the trailer.
That alone helped make Joker so noteworthy - the contrast helped to ensure that people would talk about Joker. Alongside "You'll never see it coming!" in the background, Joker's status as a mainly PS3/4 character and Reggie's "emblematic" comment afterwards.
It got people talking about Smash too, since Joker being FP1 was a major league surprise. So, yeah, I'd say that Joker helped Smash out as well in terms of sales. It was bold.