Reading and thinking as I try to get to the point you're asking for OP so bear with me?
Toxic masculinity - if we're talking only about male/male based interactions then males policing each other for sufficient masculine traits and males who don't meet those standards are rejected, mocked, and can be subject to verbal and physical abuse even by strangers on the street if perceived as presenting insufficiently masculine according to the watchers' standards. Intolerance of anything perceived as 'female' and therefore weak and inferior: camp males, males wearing clothes or engaged in activities perceived as insufficiently butch, showing weak emotions or betraying too much sensitivity. Males policing male children in the same way, with rejection and active attempts to discourage and correct, sometimes with punishment, male children showing traits associated with femininity - no son of mine plays with dolls etc. Males demonstrating displays of power and sexual intimidation to females as a joke or status increasing act among other males: whistling, commenting, harassing or physically assaulting strangers, objectifying and belittling females in conversation, talking in terms of preying on females sexually and achieving what is wanted, where males not getting emotionally involved and just seeking their own sexual satisfaction, and showing a lack of responsibility and respect in their relationships, or proving their dominance and control over a female (she has my dinner on the table at six or else) is considered higher status. Dick waving competitions in male dominated work environment where the more ruthless, aggressive and moral-less a man is the higher the respect he gains from other men.
Pretty grotty all in all. It's about maintaining and exerting power over other males to keep them in line, which is in part about males seeking higher status among themselves but also about keeping the absolute power over female people while reinforcing the message that female is weak, submissive and subhuman, to be taken and used as wanted, while being expected to serve or face punishment, and casting out anything seen as associated with or bleeding into this subhuman slave class.
Mean girls? It's not policing femininity or really working along similar lines to the above at all.
What I think you may be thinking of are the status battles that can take place between females - and nothing like on the scale or normalised culture of toxic masculinity - that are inevitably going to happen among a disempowered class living in a world dominated and controlled by the powered class. Where power is hard to get, brings status and rewards not usually available to that class, and often is about 'staying in with' and being rewarding to the powered class, that power is going to be fought over for whatever scraps can be gained. Which means showing the powered class you are on their side and a good girl, by being very good at rejecting and punishing the subordinate class - because that's what the powered class does, and the better and more vicious you are at it, the higher your status.
So females who fight their way up into male dominated groups stay there by pleasing males (saying, doing the right things, behaving in approved ways) and pleasing includes showing rejection and antipathy towards other females. And resisting other females getting into power too, because that's a threat the power you've managed to wrest for yourself. Underlying which is probably the programming that attention from powerful males and status with powerful males is a rare and prized thing, and what idiot would want to dilute and share it with someone who might win that male away?
The MiL who was a put upon, bullied drudge permitted no feelings, the worst of everything and to suck it up without complaint as a young wife and mother by older women in her family. And is now taking her own turn to have the power and release of bullying her own DiL and being the high status one who gets to do it.
The group of girls who gain status among each other and share a group identity by bullying, teasing, being abusive and pushing at the boundaries of their own box of 'be nice, be kind, ladies don't do that' and feeling power.
I don't think it's the same thing at all.