Is money earned from YouTube videos that contain music—even though the content itself is not unlawful—considered unlawful money, and does this ruling apply to the majority of content creators on YouTube who use music?

It is not permissible to use music in permissible clips, as it removes the blessing from the work and brings sin upon its owner during his life and after his death; because whoever calls to misguidance bears a burden of sin equal to that of those who follow him. Furthermore, the inclusion of music draws evil to the Muslim by causing him to fall into sin. As for the financial gain from these clips, it is permissible if it is in exchange for the content and not the music, but the sin of including music remains.