There is often confusion about the two. Besides the complexity vs. simplicity, there is an aspect of intelligence. Dumbing down overlooks the latter but overemphasizes the former.
It turns out the need to dumb down often has nothing to do with a solution that makes things easier but better.
A solutions that:
Activates human creativity instead of suppressing it
Provides a better experience to inspire than boreout (emotional deadening due to being under-stimulated)
Offers connection and clarity instead of confusion and ambiguity
When a situation calls for dumbing down, it's the cry for a change - something simple and intelligent (elegant).