The K-pop lyrics are either informal or formal. There's no rules to write lyrics in Korean. I don't know if lyrics would help improving your Korean but some people believe it does. I personally think that K-drama would help more since that's how I learnt English.

Adding onto what wuimy said, the majority is informal, but artists do occasionally switch between informal and formal in the same song. The best example I can think of is AKMU (Akdong Musician), as they do this VERY often.

I think the majority is informal. Anyways, there is this blog and they break down the lyrics for you and even explain some grammar points. It's really interesting. I personally use it when I want to understand not just what they're saying, but the poetic structure. I'm still a beginner but as someone said down below, I think kdramas are a much better resource for learning. koreanthroughkpop.blogspot.com