I think I prefer cartoony characters over more realistically proportioned characters because I like the stylistic clash that happens when, say, Mario fights with Sonic. Both of them are cartoony, but they were still designed with different philosophies in mind (like, Sonic's noodle legs and single eye would not be on any Mario character).
To be fair, realistic characters can and usually are also designed with wildly different styles (compare Shulk's concept art and in-game appearance in the original game to Snake in, say, Twin Snakes or Guns of the Patriots, and it's day and night). Brawl, and to a lesser extent, Sm4sh did a really good job at translating the differences in art-styles - you wouldn't confuse Pit for a Fire Emblem character, for example, because Marth and Ike had much sharper facial features, were taller and more slender, etc. And Snake in Brawl looked as realistic as a human could on the Wii. But I feel that Ultimate really dropped the ball with an incredibly homogenized artsyle for all the human characters. Like, you can't really see a difference in design for Little Mac, or the Street Fighters, or Cloud, they all look like they were designed by the same person and that really takes out a bit of the uniqueness.
And the trend toward human characters in Smash is sadly just a representation of an industry-wide trend. Most Western developers switched to realism when the shooter genre took off, but Japanese devs have also gone down the same path, just with anime human designs taking over. It might be a lack of creativity, or maybe it's for marketing reasons as audiences see anything non-realistic as childish. Or it might be due to the downfall of the platformer as a reigning genre aside from the established icons and Indies with less to lose. But just like I don't blame Sakurai for representing unfortunate industry trends with less-than desired representation for women and characters of color, I can't blame him for the exclusion of cartoony characters when that's also an industry-wide tendency. At the very least, both he and Nintendo as a company have done a great job at keeping that side of character design ingrained in their ethos.