Minority opinion or not, I find the talk and meta-analysis of a character being 'toxic' is useless and ultimately only makes it hard to accept these characters as fully realized and fleshed out - and that's something that Kishi has been very good at with quite a number of the characters in the series. Kaoru has especially been fleshed out, I think, and Bou-chan is moving up those ranks very quickly.
And that's why this chapter was so great, actually. Mask's admiration for Onibi has always tainted by an undercurrent of jealousy even while she clearly had a genuine respect and love for her friend. To see it all laid out, to hear it from her own lips, shows us just how much the envy has become unbearable. And to see it contrasted, in a way, against her view of Kaoru - who she feels closer to now, almost ironically, because of her actions, while Kaoru is seemingly trying to at least sort-of-kind-of become a better person...it's a really great dramatic shift and character development moment.
I think it's hard to say whether either of them, Kaoru or Bou-chan, are 'bad' people. Rolling_Loli pointed out fairly correctly that Kaoru's basically a Himbo (have we not gone all the way back around now to her basically being a value-judgment-less bimbo?); the worst things she did are fairly far in the past of the series now (her temptations towards Shizuka when she was first getting with Mio, and even further in the past, her actions following their breakup if you think that she was manipulating Shizuka and not really horribly depressed (not that i agree with the thought that she was manipulating Shizuka, but I could see the argument being made)). And Mask has done little 'wrong' outside of maybe a kind of coldness towards Kaoru - an understandable coldness on her part - or now, this awful little wish of hers towards Onibi, which, even in its awfulness, I feel is almost cute, almost wonderful, because she's taking it all so seriously and she's still, by her actions if not her words, blatantly full of admiration for Onibi...I don't know, it's all so great. Some of the best development in the series is definitely happening in and around Kaoru and Mask.