A few things here (I ended up rambling a bunch, so it might seem like I'm shouting angrily at you, but I swear I'm not lol):
a) Goeido manipulated Nene into doing that (yes, Nene accepts responsibility for her part in it, cause she says she did go along willingly, but still...).
b) You're making "getting with someone to make the person you like jealous" a much more machiavellian thing than it is. It's not a nice tactic, of course, but it's a common thing that people do to try to get others to notice them.
c) "Not telling lies" isn't a measure of character (in fact, Hime lies as much as she breathes and she still manages to be a decent-albeit-flawed person who shows a lot of care for others). And Kanoko is waaaaay more manipulative than Nene was back then, it doesn't matter how "honest" she's being (and is that even true? I feel like she's told plenty of small lies to Sumika).
d) Nene never tried to "monopolize Sumika's attention", she just wanted Sumika to look at her romantically (something Sumika probably already did, but didn't realize, otherwise she wouldn't have been that jealous). In fact, most of the dislike towards Kanoko is exactly because she's the only one playing the "monopoly" game (not the board game lol).
e) Not mentioning that she'd spoken to Youko is actually a biiiiiiiig thing, cause Sumika (her girlfriend, in theory! And her confidant!) abhors her and specifically told her not to do it.
People are much more on Nene's side, with good reason. Nene has a much more "normal/healthy" love for Sumika than Kanoko does for Hime (eg Nene doesn't want Sumika all to herself, exclusively, she doesn't treat everybody else as "obstacles") and, yeah, there's also the fact we're talking about Nene's/Kanoko's relation to Sumika and Nene loves her, while Kanoko does not and just wants her close for as long as she's useful to her in getting closer to Hime and removing Yano as an obstacle.
Doing something bad to Sumika cause you love her and did things in a bad way (not for bad reasons) = forgivable
Doing something bad to Sumika cause you have a bad obsession with someone else and you don't even care about hurting Sumika = much harder to forgive
Finally, there's also the fact that Kanoko actively wants to destroy most people's actual favorite ship: the main one. So it's pretty easy to see why people aren't very lenient towards her. What ship is Nene trying to destroy? She could've been trying to destroy Sumika+Kanoko, but she's not even doing that, cause she's too nice.
TL;DR: Nene is a good person who did something bad for understandable reasons (love and being manipulated). Kanoko is a bad person who did something bad for a bad reason (obsessive and excessively possessive love) and doesn't even care about the person she hurt.