Well, Lapis is painfully aware that her affections are harmful to Natori and is willing to push her away when her plan starts to save her, making it somewhat healthy, ironically enough. A true toxic relationship would have Lapis care only for her desires without regard for Natori's wishes or well-being, and the goal of the story is to stop Lapis' plans anyway.
Diana on the other hand comes off as the more toxic of the two, making a very interesting subversion of the usual love triangle dynamics.