"Open source" is irrelevant here; the issue is whether services are part of the surveillance economy. Google search is sometimes better than DuckDuckGo, possibly because they can afford more crawling, and sometimes worse because you need to try to out-smart their algorithm to escape your personal filter bubble. Neither company is "open source," but DDG has a simple, non-invasive business model: they show ads based on search terms.

"Open source" and "walled garden" are orthogonal.