If we are talking relevance it's also worth noting just how persistent of a presence Toad is, especially in comparison to Jr or Rosalina.
I mean Rosalina has become a regular in spin offs, but that's pretty much where it ends, outside of a Battle Card cameo in Paper Jam, a Mystery Mushroom costume in Maker and a 8-Bit Sprite cameo in Odyssey she really hasn't show up in any significant capacity in any of the non-Kart/Party/Sports/etc. titles since 3D World.
Jr by comparison is still a big part of the series, but even then he still occasionally misses out on major titles, such as Color Splash, Super Mario Run, the ''3D Duology'' or Odyssey or has no plot relevance in something Like Mario & Luigi Dream Team or Minion's Quest.
By comparison Toad is pretty much always around, Treasure Tracker being an obvious major sticking point, especially since Nintendo seems to hold it in pretty decent regard, given that we are getting two ports of it. There's also Run and his escort missions in Mario + Rabbids, not to mention his supporting appearences in Color Splash or Odyssey. Even before Smash For we also had things like Luigi's Mansion Dark Moon, where Toad appears, when even Peach is absent, plus even Sticker Star or Paper Jam, which don't have a ''The'' Toad, still feature Toads as the main NPC type.
If we want to talk other Toad characters then I'd also argue that Toadette is probably the biggest breakout Mario character in recent times, given that she was a spin off created character, who didn't even get any appearences between 2009-2012, so she had a lot going against her, even with the appearences in TTYD or her intro cameo in Galaxy.
And now look at her, she came back in 2013, shows up more then ever before and get's roles in non-spin off games left and right, including Paper Jam, a costume in Mario Maker, Mario + Rabbids, Super Mario Run, Odyssey and even getting to be the co-star of an entire game with Treasure Tracker, which is something that Rosalina, Waluigi or many others can't claim.
Long story short: the Toads are a huge deal, there is really no excuse for them to still lack playable representation in Nintendo's biggest crossover series.