Honestly Crash is pretty easy, only few levels actually present some challenge. DKC franchise is much more harder than that, and that is excluding all the bonuses stages, which I would consider in the hard category.
Now to think about it, the hardest part of Crash was the really bad controls for the first one, which I get would be annoying, since it really isn't on you.
I think a little differently, the SNES DKC games all seem very easy but Returns and TF seem harder than Crash to me. I think it's a matter of preference and time dedicated to each one though, I'd definitely put Crash in the category of challenging platformer along side Mario, DK, Rocket Knight, Sonic, Wario, Yoshi, Clockwork Knight and many more old platformers. None of them are very hard for the most part, but compared to much easier games that hardly even allow failure they definitely need some practice and will require a player to cope with failures before they learn how to progress. Not that I'm knocking easier games like Spyro, Banjo, Ratchet, Uncharted etc but they are played very differently.