Skopelos isn't necessarily an island to avoid, it's just horribly difficult to reach from Santorini. There are no ferries between the Cyclades and Sporades island groups, so it would be necessary to go back to Athens and take a flight to Skiathos and then a short ferry ride to Skopelos, or go by bus from Athens to Agios Konstantinos and a ferry from there.

I believe many of the takes from the movie Mamma Mia weren't even on Skopelos, and it wasn't exactly a memorable film anyway. The above post I quoted reads like an advert for the hotel in the link.