You should focus on one genre of music. Don’t try to do it all - stick with the genre you love and learn all the intricacies and idiosyncrasies. People will feel more confident in trusting you with their music if you specialize in their genre.
Finally, don't attempt to mix boring songs. If you can tell that the artist's music is not up to standard, it's best to kindly decline their offer or, at the very least, refer them to a different engineer who may be better suited to help them. Only take on clients who submit music that you are excited to work with and believe will benefit from your particular expertise.