I have thought a lot about this topic. There isn't a maxim that works for everything- some shows that adapt books or films are great, some are not. some original shows are great some are not. there's not one type of adapted work that is sure to be great or terrible.
There is a certain je ne sais quoi a musical has to have to be perceived as "original" or whatever and I think it comes down to whether a person thinks it was produced "for the art" or "to make money"
For example, Dear Evan Hansen is technically totally original, and Natasha Pierre, and the Great Comet of 1812 has source material. But ask 100 theatre fans who got caught up in the 2017 tony awards their thoughts, and Great comet may come across as more inventive, more appealing, more new, more original.
A lot of musicals have source material because creating the story is the hard part.[ and producers don't like to invest in something that's unproven.] This isn't just true on broadway, it's also why marvel is spitting out content faster than you can say "taylor swift jukebox musical".
Some of my favorite shows have source material- Aida, Ragtime, Bridges, Fun Home. Some of them are totally original- next to normal. Is Sunday in the Park with George original? Or could you argue the source material was a painting? If so, it's the most inventive type of source material. Is Chess original? or does it being loosely based on certain real-world chess stars take that title away?
Personally, I think an original score is more important than having no source material.
Last big hit original musical, no source material: Dear Evan Hansen, and look at its reputation now.
It's ok to adapt works to the stage, iconic shows and moments are created when you do. But when art/ theatre [at least in the US] isn't subsidized, commercialization drives creation, and original work, ideas, and new artists suffer, and not just in the theatre. TV is changing, and new work is thriving outside the constraints of network tv [not to say that there aren't good broadcast shows]. There just isn't a well funded theatre equivalent yet.