“Mullvad Browser was developed by the Tor Project who have a proven track record of building and deploying free and open-source privacy preserving technologies such as Tor Browser, Onion Services, the Tor network etc”
Tor made the actual browser, not Mullvad.
If you trust tor then you already trust Mullvad Browser as they’re from the same people.
“Unlike other browsers on the market, Mullvad Browser’s business model does not rely on capitalizing on users’ behavioral data. Mullvad makes money by selling their VPN, they are not in the business of selling user data from the browser”
From same link.