I recently re-watched Moonstruck. While not total junk, the material is very very slight. Yes, it could be a musical but it needs a stronger plot, otherwise it's indistinguishable from so many other love stories.
I disagree with you on several levels. First of all, I wouldn't use the word "slight" to describe the material. It's a light comedy, and was intended to be a light comedy, but that's not the same thing as "slight." There is a lot of passion in the story, and that's the source of much of the comedy. And finally, a movie like that is arguably the perfect source material for a musical, because if the songs are well written, they would only add to the emotions and the comedy of the source material.