That said, I took a brief look at the API documentation that you ...
The code is specific to google, so what you're trying to do won't work. Here's something you can try based on the original helm-google-suggest command.
(defun helm-ddg-suggest-fetch (input)
(let ((json-array-type 'list))
(format "https://duckduckgo.com/ac/?q=%s" input) t)