It's not deception if you tell people you're doing it.

DDG massively oversells their own contribution to the search results.

If you look at their marketing material, they claim to "use a variety of sources, 500+ to bring you your search results". This is of course technically correct, but the truth is they are a relatively thin wrapper on Bing results.

So yes, I'd claim it is a form of deception.

Taking someone else's product and making it better is a useful service.