There is discussion about this on this ticket: DuckDuckGo Onion should be the default instead of DuckDuckGo (#21483) · Issues · The Tor Project / Applications / Tor Browser · GitLab
Before that is going to happen we’d need some data on whether DDG can handle our load and how large the performance drop-down for searches would be.
As mentioned in the ticket, this is a very important point to keep in mind.