I think the reason why pretty much nobody was mad or upset about K. Rool is because he's this perfect balance almost of many fan requests, he's goofy but also a threat, he's a proportionately lesser known character but an important part of an iconic series, e.t.c.
Ridley was a decisive choice, which is probably why people don't talk about him as much as they do K. Rool - Ridley had tons of detractors, and some of them had pretty valid arguments, even if they were all proven wrong by Sakurai himself, while K. Rool had some but most of them were just GameFAQs users looking for someone to hate on - another factor that might play into Ridley being forgotten is mods: prior to Ultimate, there were no decent K. Rool mods for PM and no major fangames featuring him in a playable role - but if you wanted a Smash Ridley, there were tons of options out there. I feel like the Ridley support was more just a really, REALLY vocal (and coding-skilled) minority in hindsight.