Popularity is both underrated and overrated in terms of being advantageous for a character's inclusion, in my opinion. On one hand, there has yet to be a Smash game where the most popular of the newcomer pool has not become playable (Bowser/Peach, Wario/Dedede/Diddy/Sonic). Out of the few that haven't, there are often special circumstances involved, like Sakurai not wanting three Mario newcomers in one game with Wario (then thinking he deserved better than to be a clone), Sakurai wanting to be modest by withholding Dedede, and Sakurai considering Ridley "pretty impossible." Considering we can't reliably account for these, it's best to disregard them until they emerge.
On the other hand, people see those characters and think that, because they were popular, anyone with even a shred of fan support is a likely character. This centers mainly around self-explanatory one-shots like Owain, but extends even up to guys like Toad, with whom people will equate a few scattered posts of "I'm a Toad fan" as him having a large supportive fanbase. Basically, people have to learn to discern between legitimately noteworthy popularity and...the rest, and then speculate accordingly. It can hinge on being a judgment call at times, but there are a few obvious outliers in my opinion.