Was the song actually that good, or did your brain simply tickle the 'appreciation for incredible beauty' neurons while playing back some Nickelback memories?
I'm pretty sure I've experienced both, actually. Occasionally bits of it, melody or words, will survive in my memory that I think are actually good, if only I could reconstruct the rest of it. Other times I'm pretty sure there was nothing actually there.
The bit you remember might be great. The part you don't remember might never have existed. I've often "solved" problems in semi-lucid sleep, by brainstorming an idea and pursuing it, but when I push, the idea doesn't makes sense, or is meaningless, not just wrong.