Tingle I generally agree with, although I strongly disagree about Tetra. Sakurai's already tried to include her in Smash (most likely) once already. She's one of the better known Zelda characters in Japan, one of the most recurring characters in Zelda, considering she's a single character from one era of Zelda that came back three separate times after Wind Waker (which is four times the appearance of any individual Impa), and had a lot of attention leading up to the fighter ballot in '15. She lead the advertising campaign in Hyrule Legends, enjoying renewed popularity with Wind Waker HD not long before that, and (although she didn't personally appear in it), probably also enjoyed some help from another Toon Zelda game - Triforce Heroes - which was released just months before the ballot began (interestingly, Triforce Heroes was originally supposed to be a sequel to Link Between Worlds, but reverted back to Toon Zelda style because it's so popular). This is all aside from the fact that she's one of the most expressive, developed, unique characters in Zelda lore, and an honest to goodness pirate (which Smash utterly lacks). Her addition would balance out the aesthetic of the Zelda roster well, which looks a little serious and Fire Emblem-y right now, while Toon Link sticks out like a sore thumb - which I don't think fully appreciate's the Zelda series' wide range of flavors and tones. Given that Toon Zelda's already strongly represented in stages, and Sakurai evidently wanted to create the character at one point - it seems profoundly evident Tetra would be a natural character to consider.
Honestly, it seems the only reason people are against her is because they dislike the idea of having three Zeldas. Which is a fair point, although Tetra looks, acts, and her entire moveset would be so utterly unlike Zelda (she's practically an anti-Zelda - she wields a flintlock pistol for goodness sake), the point appears very strained to me. I do really want Impa to be on the roster - she's definitely earned a spot - but I don't think she adds nearly as much to Zelda's representation as Tetra would (what other franchise could boast introducing both a ninja AND a pirate?), and her strongest argument for inclusion seems to be that she's had such a recurring role - a role Tetra has four times the exposure of for any single iteration. Had they both been one-offs, I have doubts as to whether Impa would be a viable contender, whereas Tetra would likely still have a strong following. Not to mention, we've already got a few ninja characters. Given Tetra's unique look, moveset options, relative popularity, and coveted role being one of Sakurai's chosen, scrapped characters (who have a habit of getting in later, if Dedede, Marth, Mewtwo, Ridley, etc, are any indications) - I think ruling out Tetra is simply silly. By Sakurai's own considerations, that she deserves a spot in the Smash Bros roster is hard to deny.