Not necessarily. Interface design is a communication medium. A skilled designer can often communicate to multiple types of users solving different types of problems. When an interface is “dumbed down,” it’s usually because a non-designer was trying to ‘clean it up’ or make it pretty by removing what they didn’t need and ignoring what everybody else needs. It’s difficult, and that’s why it’s a distinct field with its own degrees and professional programs.

Sure, but whenever you can make life easier for those who have are having hard time without making life harder for those who aren't, you should do it.

For example, no power user benefits from having to use work arounds to make a product function. It's an annoyance to them, and it's an insurmountable obstacle to others.