I thought I’d post a quick explanation of why this response is mocked. There have been many explanations like this, but this is mine.
Anyway. Let’s have a look at a model scenario.
Person X speaks of a personal experience of misogyny and/or harassment to person Z on twitter, “why can’t men leave me alone.”
Person Y sees this and interjects, ‘but not all men!’
This is the wrong response, and eminently mockable. Why? Well, the obvious ironic rudeness of the interjection itself: a man thinks that it’s acceptable to butt into a conversation to sideline the concerns of a woman who has experienced male oppression with a recourse to his own supposed victimhood. It’s not just rude, but hilariously and infuriatingly emblematic of the problem as a whole, and indeed further legitimises responses of ‘YES ALL MEN’
Put it this way, if you feel the need to qualify someone’s experience of misogyny with a personal cry of ‘But I’m not! So not all men!’ then not only are you demonstrating the issue by trying to make misogyny about yourself, but you’re concurrently implying that the majority of men are the problem. Else, why would you feel the need to claim exemption?