The legal term is "deception".

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.