For reference, I hover around Level 60-61.
I think the ratings don't always tell the full story, but typically, when I'm automatching, I expect players at Level 53 or above to be capable of outplaying me, but it isn't really consistent unless they are at least Level 58. I would estimate that most players at Level 50 or so are 'good' at Dominion. Below 50, there is just a lot of variety. On my alt account, I have seen players in the 40s play like some of the best players and also players who didn't seem to understand key cards. So I'm going with a rough estimate of Level 50 as a 'good player.'
Still, it's a card game and sometimes the best players even miss a good strategy, so really anyone can beat anyone. You just can't judge by individual games or ratings at times.