If the script was uninspired and nothing special, and the filmmakers just saw this just a sequel "product", there's no way that the actors could save it.
Totally disagree. The third one was a cash in. The second was done with a lot of love and attention.

I even thought John Wesley Shipp did a great job, at least considering the stereotypical and somewhat bland character he was written as. (Still the best character arc in the film, oddlyBastian's was a bit confused and just too rushed and poorly done). Even Brandis, who mostly overacts, has a few genuine moments with Shipp.
You could tell Burt was just having fun with the role. She probably knew it was dreck and just wanted to give a deliciously hammy performance. I don't fault her in the slightest.