Kan is just being honest, as far as I know he is one of the better persons in their friend group. He doesn't hide his feelings and makes it very clear that Miwa is not helping herself by hiding who she is, it may be harsh but Kan is just telling the truth.
He is the the kind of person Miwa and Saeko need, he may hate everyone, but he hates them equally. For him every person is the same, anyone can be a bother to him. Yeah it may be a immature way of thinking, but he's barely an adult and, it doesn't really matter cause he doesn't discriminate.
Thank you so much by the chapter btw <3<3<3<3<3<3<3