You know, whether a character is "well-deserving" is an interesting discussion.
Mainly because the Smash series has introduced me to a lot of obscure games and characters, (some with their last titles that came out looooong ago, with no recent games) that I never would've heard of otherwise.
I think Smash isn't just about "well-deserved" characters, but also a way to introduce the people to new franchises that they might like.
Brawl's Snake might seem more of a natural Smash character now that he's been in one, but let's be honest. I never would've thought he'd actually work IN a Smash game if it wasn't for Brawl (and especially P:M haha). I mean, come on, this super manly dude who goes around snapping necks and sniping dudes in the head while smoking cigs and going on long discussions about the meaning of life is going to show up in SMASH BROS?
HOW DOES THAT EVEN WORK IT DOESN'T FIT AT ALL AAAND HE'S 3RD PARTY BLAHBLAHBL-
Fire Emblem was another one that I was introduced to because of Melee as well.
Point is, "well-deserving" can be a criteria for Smash rosters, but it isn't the only one.
Anyways, as someone who's finished the first Shantae, I can tell you, she's actually a very charming character with an equally charming world to navigate. Her first game is probably the most beautiful Gameboy COLOR game I'd ever seen in my life. And it kind of feels really bad when people dismiss her without even looking into what she's about at all.
Not to say whether or not she'd work in a Smash game, but no need to slander the character just because she isn't the character you'd want.