If you honestly think there's a ton of people who are going to be hyped about the likes of Takamaru...Balloon Fighter...Mach Rider...
We never said "hype," just "well-received," which is actually a pretty low bar to clear for casuals.
Actually, most people are well past the point of complaining about retro-type reps like these, especially ones that fill popular fictional archetypes like "samurai warrior" or "motorbiker" that we don't have yet.
The ones that would ACTUALLY be "well received"?
Isaac: No (perfect example of a character that'd be huge here, and nobody outside of SmashBoards gives a **** about)
Dixie Kong: Yes
Cranky Kong: Maybe
Funky Kong: No
Toad/Captain Toad: Yes
Pauline: Yes
Waluigi: Yes
Bandana Dee: Maybe
Marx: No
Adeline: No
Tom Nook: Yes
Ring Fit Trainer: Hmm...that game sold really well, but I question how much crossover there is between it an Smash. Wii Fit Trainer's reception was pretty mediocre.
Impa: Yes
Skull Kid: Yes
Midna: Maybe
any of the BotW Champions: Depends on the Champion
Lyn: No
Krystal: Yes
Slippy Toad: No
Sylux: No
Raven Beak: No
Takamaru: No
ExiteBiker: No
Balloon Fighter: No
Mach Rider: No
I'll do my best with some quick words on these.
For Isaac, people not knowing about him does not mean a poor reception, as we've seen time and time again with characters like Shulk or Little Mac. He wouldn't be added to target casuals anyway, though: he'd be a bone thrown to the fans. As an additional note of personal perspective, I think he's a character that some need to really see in action in order to understand his appeal, with all the unique abilities he has.
Funky Kong appears in casual-littered games like Kart and Super Sluggers, plus DKC.
Marx, appearing in Super Star and Super Star Ultra, along with having a playable appearance in Star Allies, appeals to like three separate generations of Kirby fans. Plus he's a boss in Ultimate which vastly elevates his recognizability among Smash players; we have Ridley giving precedence to bosses becoming playable now, so I think if he got in a lot of people would make sense of it in terms of that.
Adeleine there's someone else who would probably do a better job of explaining than me. An N64-era retro rep from an existing series sounds a lot like K. Rool, though, for someone who doesn't care about popularity among hardcore fans on Smashboards.
Wii Fit had some negative reactions but these days it's pretty well-accepted as part of the roster; if anything Ring Fit would be more welcome now than Wii Fit was in its time since it wouldn't be as much of a shocker, nor would it change the way people think about what types of games get represented.
With BotW Champions you would of course have people upset that their favorite didn't get in, but nobody would question the game of BotW itself getting a dedicated rep, and most would quickly understand the fact that only one could get in just like they did with Byleth over House leaders or starter Pokemon.
Lyn I could see some negative reactions in an EiH scenario, but if she came with a reorganization of the FE roster in general it wouldn't be bad (certainly not among FE fans, which are the people her addition would be targeting anyway). Many casual Smash players at least have seen her before, too, since she's been an Assist for a long time (and a well-liked one at that, I think).
Slippy Toad would probably be perceived as somewhat of a joke character by some casuals, but Star Fox 64 is still as well-known as ever at least within Nintendo circles. There would be some haters but most would be charmed by his goofy demeanor and possible Smart Bomb shenanigans.
Sylux and Raven Beak depend a lot on how Dread's legacy holds up and what happens with Sylux in the Prime series. If Sylux ends up a main villain of Prime 4 or something, then watch out.
I argued for the 80s retros above.