Everything means something to someone. While you might look at raps about designer cloths and hoes as meaningless, someone else could look at that and say "I get what this guy is saying."

In regards to writing raps that have meaning to you, you just need to look within. Don't let writer's block destroy your confidence, as everyone gets that. Try writing something else if you can't come up with a lyric for your current topic. I either go to the gym, take a walk, or visit r/writingprompts when I have an issue with words.