On the few Japanese sites I visited, I noticed that Waluigi and Bowser Jr. were the most requested Mario characters, with Rosalina, Daisy, and Geno having a decent amount of requests. Paper Mario and especially Toad didn't appear very often. I think the general consensus on Toad is that while he is a very popular Mario character, people either don't expect him or don't want him playable in Smash. As for Dr. Mario, he definitely got a few requests, but he was mostly requested as just a Mario alt. costume. Keep in mind I only visited a handful of sites, and I don't know their "Smash credibility" (like if they were equivalent to Smashboards, GameFAQs, IGN Boards, or Youtube comments lol), but these were the overall results I found.
As for the the popularity of Mario characters overall, they definitely weren't requested nearly as much as Mewtwo or Roy (like SSBF said), nor were they discussed or requested as much as new FE, Mother, Pokemon, 3rd Party characters, or individuals such as Shulk and Palutena, but there were more requests for Mario characters than Zelda, Kirby, Metroid, Star Fox, etc. ones.