I don't think that challenging small instances of sexism trivialises feminism. Instead I think it meets people where there are, from what they see and experience and can change and helps raise awareness of the wider issue of sexism and oppression. It's not a zero sum game.
It is clear that sexism which risks the life and bodily integrity of women is urgent and important and worthy of concerted efforts to prevent. But that doesn't mean small ways in which women and girls are overlooked, diminished and undermined are undeserving of time and attention. It all adds to the culture we live in and I think it can help.
If small girls aren't called bossy when they are assertive, then maybe they will be leaders of the future who can have real power to change things. If a small boy isn't told that the worst thing in the world to be is 'like a girl' then maybe he won't grow up to consider women and girls beneath him. If our media stops telling people that 'woman' is a personality type, worthy of only one character or panellist in male dominated shows then maybe issues which affect women would be treated as important issues which can affect half of humanity, rather than a special interest specialism that can be largely ignored.
It all matters. It's a straw man to say that feminists aren't tackling the big important stuff. Saying that feminists should only concern themselves with the things which risk lives, safety and livelihood is holding them to a ridiculously high utilitarian moral standard that means you can only act to help those in the very greatest need. It isn't how most people operate.
We don't tell doctors to stop treating minor ailments just because they are not life threatening. We don't stop helping people in poverty in the UK just because they aren't as poor as the poorest people in Africa. We don't stop treating people with benign growths, just because other people have cancerous ones, and in turn stop treating adults with cancer because it diverts resources away from children with cancer.
There are loads of non-critical things we spend our time and resources on. Preserving old buildings of dubious historical importance. Playing computer games. Reading stuff on Mumsnet. Writing articles victim blaming women for still being oppressed. There's no reason why we can't spend a few minutes adding our name to a petition against some small instance of sexism, is there?
So we have:
The Straw Man fallacy - because feminists do work on the 'important stuff'
Two-value logic - Cause X is important. Cause Y is not Cause X.
Therefore, Cause Y must be unimportant.
A false dichotomy - because you don't have to choose between Cause X and Y
A fallacy or rhetorical device that I don't know the name of where you say you shouldn't do anything about A until B is solved, which you can't start work on until C is solved, etc.
All without saying that possessing a phallus is a fallacy in feminist debate.