Mullvad didn't donate money. One of their founders donated some of his private money to this loonie party. Money that may have come in full but in all likelihood really only in part from the payments he received from Mullvad. But it wasn't a company decision to give them money.
Now, Mullvad, the company, in my opinion reacted poorly to this. It's not so much the donation but the company's reaction to the affair that would've been enough for me to switch VPN providers. I wouldn't hold them responsible for the decisions one of their leaders made in his private life. But this "all politics are okay for us" PR response was the wrong reaction for me.
The party is better described as a horseshoe party. They present a mix of politics that's mostly very left leaning except for their views on immigration where they turned hard the other way. Calling them Nazis is using the term a bit inflationarily. The real Nazis in Sweden are the Sweden Democrats and they are already in parliament.