I doubt Sakurai hates Ridley, but it's obvious that he's not a big fan of the Metroid series. It took us until Brawl to have a stage resembling an actual Metroid-series location, and even then none of the stage concepts were actually Metroid-esque until boss battle fest Pyrosphere in Smash 4 - for a series that is most prominently
about exploration of its locations. During the development of Brawl Sakurai wanted to add a second Metroid character and the best he could come up with was Samus, a second time, acting completely OOC and with a moveset that was 95% made up.
I would suggest making him play Metroid Prime 1 all the way through to change his mind, but then again this is the same man who likes George Lucas. So no accounting for taste y'know.
I personally blame the games more then I blame sakurai. Ridley needs more development. How about that manga everyone brings up, put something like that in an actual game. People hate on Other M but I liked seeing Ridley change forms.
Why would character development matter tremendously for a character to be in Smash Bros.? The headline fighters of the series are Mario, Link, Pikachu, and Kirby.
Even if Corrin didn't get in you won't get Shovel Knight. Hell, I'd argue we'd not get anything at all.
I'd say we would definitely have gotten
something for the dev time of Corrin, even if it was just Wolf or somesuch.