Mullvad Browser:

Your browser has a non-unique fingerprint

Within our dataset of several hundred thousand visitors tested in the past 45 days, only one in 1041.72 browsers have the same fingerprint as yours.

For comparison if I test in Firefox (configured as I was previously using) I get:

Your browser has a unique fingerprint

Your browser fingerprint appears to be unique among the 304,204 tested in the past 45 days.

"Strong" protection means that your browser is configured to protect against tracking, both come back as strong here, but as far as I can tell that doesn't take into account tracking through fingerprinting. You're right, you don't want a unique fingerprint.

When you customise and add extensions to your browser, you add to its fingerprint so even privacy extensions can be counterproductive. The way Mullvad Browser works is that because it comes ready configured for privacy with various privacy extensions (including UBo) installed by default you don't have to make changes. While this doesn't give you a "common" fingerprint exactly (hence "only one in 1041.72 browsers"), it makes you look like all the other Mullvad Browsers so there's safety in numbers. There are a few more nuances to it than that but that's basically the idea, and it's a balancing act, you could be less unique than that but you would need to make other compromises.

The catch is that you shouldn't try to configure Mullvad Browser yourself since that negates the benefit; you have to be able to live with the browser as it is out of the box which may mean sacrificing some of your favourite extensions.