It all depends on your interpretation. Having just stage managed one production and acted in another I can honestly say that most people, including the Catholic audience members get a big kick out of it. The show started as a joke/skit and grew into a full fledged show. It is meant to be fun and for the most part it is not meant to be taken seriously. There are a few moments in the show that are serious, but those are few and far between.
I recommend renting the DVD if you can find it, Netflix does not have it, I had to buy it from Amazon. If you are able to get a copy of it keep in mind that your director might change stuff. For instance I was in it at a Christian theatre and a some of the words or jokes(damn, hell) were altered. In the one I stage managed nothing was changed. Plus I'm not a dancer and I couldn't do pointe so I was able to do the Dying Nun Ballet on skates, which turned out to be one of the audiences favorite scenes. Who knew roller skating could be so impressive?
I'm not sure if you're looking into it for directing or acting purposes, or just to watch a production, but the show is a lot of fun, very high energy. It has a lot of songs and can include a lot of choreography and with only five characters there isn't a lot of down time. However if you're a comedic actress this is a great way to show off your skills.
Currently~ Miss Myrna Thorn in Ruthless! the Musical
Recently~ Emma Carew (Jekyll & Hyde), Sister Mary Leo (Nunsense), Demeter (Cats), Tzeitel (Fiddler on the Roof)