Well first it of all it wasn't that bad, certainly it wasn't in the same level as let's say "Cheers" but this clearly had a younger audience in mind.
In the 1980s shows aimed a kid audience were popular including "Small Wonder", "Harry and The Hendersons". This was on ABC for almost a year so clearly they must have liked to keep it on that long.
Some of the jokes and plots would still work today like the "Charmings go plastic" episodes where The Charmings like other Americans develops a sick infatuation with credit cards. It eventually results in them being broke and going at it. Other episodes like "The Witch of Van Oaks" with John Austin were good.
ABC around this time had shows like "Perfect Strangers", "Head of the Class" and "Full House" so "The Charmings" was meant to be as successful as those shows. Unfortunately because of poor ratings and having such a bad timeslot it never did last.
It's a funny odd sitcom for the family. If you're into those you might like it but as I said clearly meant for younger audiences with the exception of a couple of episodes.