'Stupid' is a poor word choice because it is a value judgment more than just a personal opinion - but I can see how some people find a lot of rap/hip-hop lyrics to sound 'stupid', meaning that they are about base or crass ideas and don't have anything particularly intellectual going on. I think the exact same can be said about a lot of pop and country lyrics. The mainstream stuff on the radio and in the charts is almost always that way (though not always, there are plenty of exceptions).
But like all genres, there is some really intellectual hip-hop out there. The whole "underground" scene (mid-late 90s into the 00s) was founded on thoughtful lyrics detailing the artist's accounts of trying to understand life and the things going on around them. I think that movement made many in the next generation of rappers much more thoughtful, introspective in their lyrics. There are household names (Kanye, Kendrick Lamar, etc.) who have some pretty deep, heady content in their lyrics. The whole sound or presentation may not appeal to everyone, but it certainly isn't 'stupid' by any measure.
Wasn't KRS1 known for being intellectual? I haven't heard to much of his stuff.
Looks like my humor wasn't wasted.