Well I didn't say my observation of the particular K. Rool fans in this very topic were respectful, I was talking about the remainder of my posts (including the rest of the one in which I made said observation).
And you are certainly exaggerating. Do you know why very few people were extremely vocal about Rosalina? Because the mainline of Mario characters were already there. We had Mario, Luigi, Peach, Bowser and the side-series characters of Wario, Yoshi and Donkey Kong. Not counting Wario as a Mario character (since he mainly represents his own 2 series more so than the Mario series), we even went down one Mario character in the transition from Melee to Brawl - Dr. Mario.
There just wasn't a lot of expectation for an additional Mario character; the last time the roster changed, it went down, and people felt uneasy about making speculations on very recent characters such as Bowser Jr. and Rosalina, while Toad was disregarded because he's very generic, even more so than Yoshi (who has a plethora of unique abilities).
Additionally, Rosalina's status as spin-off character was all but secure early on. She got skipped in a Mario Tennis game because of time restraints (Luma got in instead) and she wasn't made a playable but a boardmaster in one Mario Party game. However, since then, she's become a fan favourite in Mario Kart Wii and especially 7.
In regards to Smash, I've seen plenty of people suggest Rosalina, though, and not only as a costume. Again, personally, I expected her to be a costume at best given the track record for Mario characters and the recency of the character. For me, IF we were to get a new Mario character, it was either between her or Jr. - little did I know, eh? However, the moment she was revealed for 3D World I knew she was gonna be playable in Smash (and I argued as much back then).
Suggesting that the character wasn't popular or didn't have (m)any supporters prior to her reveal is being in denial. They just weren't as vocal or didn't have as many expectations simply because the character is both recent and from a series that had been reduced by one character before. Nobody truly expected a Mario newcomer, let alone 2, and Dr. Mario returning. That's why the discussions were fairly quiet about her, and Mario characters in general.