No.

You are not donating to a cause when you purchase goods or services. You are not liable for what someone else does with their money.

Boycotts are only effective when paired with direct action. There is no direct action in this case.

Boycotts that involve not using some necessary service are paired with replication of service through the group organizing the boycott. There is no replication of service in this case.

So you aren’t responsible for the actions of the mullvad owner and not using it isn’t an effective method of changing his behavior (there is no direct action or service replication to accompany an organized boycott, which I would also argue there isn’t since people who made the decision to switch didn’t do so in a unified way or with any demands).

But does it protect you to leave mullvad?

No.

You’re leaving the most straightforward, private and secure vpn with if not the best apps, certainly best in class apps, and a history of literally functioning as a brick wall against investigations by the authorities. No other provider has done what mullvad has done when the screws were put to them because no other provider has had that happen.

So it accomplishes nothing and makes you less safe.