If I have to customize, customizing Mullvad will give me better anonymity than customizing other browser. Plus I can always switch back to the standard Mulvad mode by opening an Incognito windows, as my customization (i.e. my extensions) don’t run in Incognito mode. I do not lose the ability to use Mullvad as a true ephemeral browser if I wanted to. Instead, it make my regular normal browsing more anonymous than I could possibly otherwise. It open a bridge to am option with more anonymity without sacrificing much usability, while still allowing me to go for full anonymity when I actually want to.
Also, as mentioned above, not all kind of customization will make me more fingerprintable. There are certain stuff that for sure will not make me any more stand out (i.e. clearing my browsing data doesn’t make me more fingerprintable), and certain stuff that can slightly alter my fingerprint but will unlikely to cause me to be uniquely identified (since there need to be enough unique things about me for the tracker to be confident it is me).
If you customize the right way and don’t do any of the DON’T, it is unlikely that you will be any more trackable in practical term.