If Mullvad fired an ordinary employee for donating to an anti-immigration party or pressured them into not donating, I would absolutely find a new VPN provider that doesn't do this over it.
For sure, and I think most people would agree with this. However, I also think most people treat it differently when it is someone so high up because they have a lot of control and, more importantly, accumulate a large part of the profit.
I don't think you should not be allowed to have strong political opinions in such a position. I just think that in this case, it is very dangerous to express so much support for them in such a public way. I say this because (from my perspective at least) a large part of Mullvad's "competitive advantage" is their brand image, and it just feels that a link to controversial politics probably does more harm than good to this brand image.
As would I. That doesn't seem like a nice workplace. I'm pretty sure it's illegal too, at least in Sweden.