Isn’t searxng more safer and better than ddg? I also believe there were talks about ddg being untrustworthy.
Mullvad browser is a fork from Tor browser that also comes with ddg as a default search engine, so I think it should be Tor browser who should change it first. However, if you want, you can just manually add any search engine of your choice and it shouldn’t affect your browser fingerprint.
SearxNG isn’t a search engine provider. It’s just the software in itself. You have to trust either yourself if you self-host it, or a third party of your choice like PrivacyGuides.
However, there’s no ″big″ enough SearxNG instances that can allow Mullvad or the Tor Foundation to give their trust to just set it as a default. It’s true, however, that in the case of DuckDuckGo, using SearxNG allows you to use their result much privately, as DuckDuckGo will only see the search request from only one centralized SearxNG server instead of individual users
(That SearxNG instance does need to have a sufficient amount of minimum popularity to really blend the search results with each other users however).
You are free to change your default search engine in the Mullvad or Tor Browser with a minimum impact on your fingerprint (With perhaps, just some already proxies, ad-redirection if you click on those, and maybe the header telling the website, where that user came from on their website.)