Underrated is a bad word... He's certainly highly rated and successful, maybe overlooked is a better word? People rarely cover his songs (close to never), he rarely goes to interviews, and Grammy forgot about Mark's existence despite the fact that he's producing one of the best soundings albums in the industry, and with great songs. People rarely discuss his music, he's overlooked in various lists, music YouTube channels, etc., heck, he's not even knighted yet, and he's probably the greatest contender for this. I don't think he will be, by the way.
To be honest, I kinda like that he has this "independent artist" feel, but sometimes I sort of wish more people would appreciate his brilliant songwriting, or at least guitar playing. I try to remedy this a little bit by discussing his music on my channel.