If ”doesn't exist” means ”has no physical form” in our Reality, then yes. You would be able to sow ideas into others, escape to other worlds on incorporeal forms and so on.
However, if ”doesn't exist” means literal non-existence (as in nothing), then it would be useless in the broadest sense since it would be equivalent to not manipulating anything. On stricter terms, the power to manipulate nothing is easily one of the most powerful in fiction only if played under very specific conditions.
There are other definitions, though it would just fill my comment with unnecessary examples when I've already exposed enough.