Impa is just not an icon though. You keep referring to her recurring status, but how major has she really been? Would you call Beedle an icon because he has recurred more than Tingle?
I mean other than the fact that she is usually the one who finds Link, or leads him and teaches him new things about the Hyrule, or that she’s the one who usually protects Zelda till Link’s ready and is basically the only reason Zelda is still alive in most games, or that she was literally chosen by the Godess in Zelda to be reincarnated making her only 1 of 4 characters that get reincarnated with Zelda and Link along with Ganon being the others. It’s pretty clear that Impa has quite a bit of significance in the series as usually the one who advances the plot. She was the one who taught Link Zelda’s Lullaby in Oot along with playing a part in sealing Ganon. In BOTW she was actually given some family, and was the first person Link was told to meet.
I do think Impa is iconic, I just think it took a while for people to realize she was important to the series as she always comes off as some secret 4th party, more a background than foreground force. Hyrule Warriors, and Skyward Sword, to an extent really brought her to the front for once, which really made people realize that hey...This character, ... she’s been important this whole time yet none of you payed any attention. I feel that there’s been sort of a shift since Skyward Sword to bring her more into the forefront as one of the driving forces in the games, which I why I think a slot in Smah is more than justified, especially since she’s now had a few younger forms and been given plenty of material to work with, especially since Hyrule Warriors has taken a hold of her.
Well, I know that Tingle has recurred, but I think you're overestimating how much MM defines Zelda iconography. Nintendo clearly wants to preserve it as its own standalone weird game, so most iconography comes from ALttP and OoT if anything.
No, most merchandise is based on Majora’s Mask. Other than generic Triforce and Hylian Shields, when you search up Zeld Merchandise there are a ton of MM specific stuff that pops up with the occasional Wind Waker and Oot. Most people seem to remeber elements from Majora far greater than anything else from Zelda with the mask and moon being two of the most prominent Zelda figures. It may be a weird game in the series, but it is hard to deny the mass appeal of images that game envokes.
But if it wasn't for Hyrule Warriors tho? Impa is iconic now but Skull Kid has since been iconic. I was incorrect about Tingle but Impa has only just got it.
I don’t believe this really hurts her though. She’s just been given better treatment in more recent installments and has kind of been brought more and more to the forefront.
Also MM wasn’t always popular, it was one of the titles that went through the Zelda cycle where it had a long stretch of time where it was panned/ not talked about by the “core fan base” Going back, it seems like Skyward Sword release in 2011 was when the game got extremely popular to make Skull Kid worthy of being a character. Really not much more time between the the explosion of MM popularity and Impa as Hyrule Warriors only came out like 3 years after the Majora’s Mask fandom really got vocal for most to start taking notice. (Yes, I know it was a cult classic, but it definitely wasn’t seen as the superior game to Oot or best in the franchise till then. At that point it was just kind of that weird title, but as we move further nowadays, basically MM and Wind Waker battle it out for best title award along with BOTW, Link to The Past, and Oot.)
Honestly speaking though, I actually do support Skull Kid in Smash as the Zelda roster should really ditch Young Link and Sheik, and instead have “new” characters based on the most popular titles. Heck, I’d even say ditching Ganondorf may make the roster better as we should just have Beast Ganon. He’s been in more titles, has a bigger moveset to pull from, and seems to be the future of where the Ganon character is heading. If I picked the Zelda roster, I’d have BOTW Link, LttP Zelda, NES TLZ Ganon, WW Toon Link, MM Skull Kid, Oot Impa, and Tingle because he does have his own series and he’d be a funny joke character.