It's this, but the "party line" on K. Rool goes back further than Retro's games. You have a small number of arcade fans who resent Rare for reinventing the character and series, period (and also the Gamecube-era fans they won over during their heyday). Not only did DK change from an old-timey villain to a platforming hero (hence the "bawww it's too much like Mario/K. Rool is just like Bowser), but the DKC series became bigger than just DK himself. A lot of the envy got channeled toward "bawwww too many characters, DK should be in the spotlight," which entailed all of the Kongs at first (see Jungle Beat), but eventually spared the Kongs who got "legitimized" through new "main series" games. Notice the tolerance of all the TF Kongs, not to mention the DKC formula, which became accepted as the needed format for DK games to actually sell (also see Jungle Beat).
The hate also rubbed off on Kremlings and K. Rool; when you have compelling recurring enemies designed with Rare's quirks (and a conflict that's not their preferred DK vs. Mario, which also explains the constant denial of Cranky as the original DK), that's going to be a target. Couple that with Retro's disuse of them thus far and you have an easy recipe for these people to dance on their graves, to sing the praises of enemy groups like Tikis who are essentially wallpaper to DK himself (what with the helper Kongs all being power-ups in Retro's games).
Naturally, when you have a vested interest in characters not being in new games, having legions of fans requesting their return and overwhelmingly dominating general consensus results in a lot of bitterness, of "why do people like K. Rool??" and, more passively, talk of how "fan votes on the ballot don't matter or might not matter."
If we're lucky, we're on the brink of all that talk getting flushed away like the [insert flushable material here] it is with K. Rool's addition into Smash, and what a relief that will be.