I don't understand why TOR Browser thought that was a good idea either. It seems extremely risky to try to make every browser appear the same and simply hope that they've managed to cover every single means to fingerprint an individual. It's a game of Whac-A-Mole where your adversary is constantly exploring new fingerprinting techniques so TOR/Mullvad has to invest their time and effort into doing the same just so they can counter them all. If they miss anything or don't catch it before or as soon as anyone else does they lose the ability to hide in the crowd entirely.
Some amount of research into fingerprinting techniques will always be needed but it seems to me that a far simpler solution would be to randomize the fingerprint for each connection. It doesn't matter if your browser fingerprint is unique as long as it's always changing. That would also make it harder to detect TOR/Mullvad users since they'll look exactly the same as anyone else with a unique fingerprint. It also gives users the ability to modify some of their fingerprint according to their needs without losing protection. For example, they could freely change their useragent for certain websites/requests while still having a unique fingerprint.
Some amount of research into fingerprinting techniques will always be needed but it seems to me that a far simpler solution would be to randomize the fingerprint for each connection. It doesn't matter if your browser fingerprint is unique as long as it's always changing. That would also make it harder to detect TOR/Mullvad users since they'll look exactly the same as anyone else with a unique fingerprint. It also gives users the ability to modify some of their fingerprint according to their needs without losing protection. For example, they could freely change their useragent for certain websites/requests while still having a unique fingerprint.