Zappa's dirty lyrics work well for numerous reasons imo.
Firstly, it was a stark contrast to what the hippies and everyone else were signing about at that time. So, it was a reaction against his contemporaries.
Secondly, everything about Zappa and his music was extreme even the stuff that may not seem so extreme. So, it makes sense to have extreme lyrics too.
Thirdly, I think the idea of having what is basically high brow music (very well composed, hard to play etc) coupled with low brow lyrics is an interesting contrast and probably made some people appreciate either music they wouldn't normally like, or lyrics they wouldn't normally like because they liked the other aspect of the song and thus gave it a fair shot. So, it kinda forced people out of their comfort zones while still being grounded in their comfort zone.
Lastly, what else would he sing about? This is especially important for someone who has so many albums. I've often thought about how a more traditional lyrical approach would work with his music, and I can't see it working. You can only sing about love, politics, and cars so much without being redundant. And let's face it, lyrics are typically the weakest part of a song because most people simply can't write interesting lyrics. So, I think his embracing an approach of everything and anything goes makes sense for him.
Well, that's all I got for right now.
