Difficulty indeed doesn't equal quality, but it does often mean it has deeper gameplay and takes skill to get through, which is something I enjoy.Meh I don't think difficulty = quality imo. Kirby is meant to be Entry and has EXCEPTIONAL titles under his belt. Opinions I suppose. I feel Mario has a solid series and DK is good if you like run of the mill precision platforming at it's finest. Zelda to me is hit and miss. I like some games and dislike others. Matter of taste I suppose but I do realize Zelda is quite popular.
However if you like those games more it can't be helped.
I only play as characters whose games I like the most anyway so Kirby characters are my main. It's sort of like how I think it's odd to cosplay at a convention as characters you don't even like/love. Just doesn't have heart for me. I personally don't just play as characters that feel good for winning. Winning means nothing to me with a characters I don't like.
I think if you like Mario or Link than play as them than. Playing with a character you like is more important IMO but I know that's not necessarily how most people play smash.
You seem to be confusing feel and love though. For example I love Link as a character the most, but I kinda hate how sluggish he feels in Smash, so I'd never choose him. It's not about winning, but how it feels to play the character in Smash and not how much I like them in their own games (though this obviously helps when on the fence between characters).