A musical and a production of a musical are two distinct entities to me, even if they should, ideally, compliment each other. The production, in this case, a stage production being a delivery system/platform.
At its most basic, the whole of a musical is its written score, just like a novel is all you need to 'experience' a complex story, "the work" is contained in the music and lyrics.
A musical can be judged on its written music and lyrics alone.
The interpretation of its score by musicians and actors is performance and an art in itself. How effectively they tell the story through song is a whole other topic but it shouldn't go into deciding whether a musical is any good or not.
It's entirely possible to love a musical but loath its stage production. In the end, it's important to not allow performances or productions or interpretations decide if a musical is good or not.
Nothing wrong with basing how one feels about a musical on a cast recording, provided it contains most of the score and the performances aren't influencing one's assessment of it.
Recreation of original John Cameron orchestration to "On My Own" by yours truly. Click player below to hear.