Zao onsen without skiing

Is it worth going to Zao onsen only for the onsen and possibly seeing the snow monster trees?

The outdoor bath was closed unfortunately.

A second option was to visit Ginzan onsen further north but not sure if that is worth the long trip from Tokyo.

With an overnight, Ginzan onsen is absolutely worth the travel time in my opinion, but not as a day trip.
I don't ski, and went to Zao onsen mostly for the rotenburo (and also because it was easy to reach, between Sendai and Ginzan onsen). The town itself was nothing special, but it was off-season, I think, so most shops were closed. Maybe it is a bit more interesting in winter.

As far as I heard it is rather Hard finding rooms in Ginzan onsen, staying in Yamagata seems not worth it.

IMO, Zao and Ginzan onsens are both worth visiting, staying a night would be a better option, especially in Ginsan onsen.