It would depend on the environment or the moment.
If it was that person driving alone out on a long stretch of an old country highway where there's no one for miles, fine.
If it was that person driving with another person riding along (who doesn't sing), then it might be rude since it's more polite to hold a pleasant conversation than to go hollering off the top of your lungs to some song that this passenger is clueless about.
If it was that person driving along, but in traffic or with people around them... it would make that driver look like an idiot moving their mouths like some retard (windows closed) or be considered disturbing the peace (windows opened).
I don't sing to music out loud. If I know the lyrics, I'll simply lip-sync. 
