When I think powerful, I think ability to do damage as well as one's ability to survive it.
His ability to control his form is impressive, but limited. He was frozen by Ras and I'm sure there are a few acids that could mess him up. He can take a beating, but dish it out?
That's where he's lacking. Circes couldn't transform Plastic Man, but she was able to imprison him without much trouble. Physically, the amount of power Plastic Man can bring to bear is probably just a step above Batman.
Which isn't too bad, but not what I see as powerful. His power is subtle but not enough to count as 'one of the most powerful people out there'.