I've received this question a lot, lol. Well, there are two things with this name.
First: it was a mistake, actually. When I created my account in FanFiction.net I had the intention of going by UnderclassHero, which is the name of an album and song from Sum 41, a punk-rock band I really like.
But as I tiped it, I got distracted and wrote Underrated instead of Underclass. I was about to change it, but then I thought about Captain Marvel/Shazam. He's my favorite superhero of all time. If you don't know him, he's Billy Batson, a kid (usually between 10-15 years old) that was granted with extraordinary powers by the Wizard Shazam, and when he says the wizard's name, a freaking lightning falls down from the sky and turns him into Captain Marvel (or Shazam, nowadays). He's basically a kid with Superman like powers, but also magic, lightnings, a cooler costume and a talking tiger (really).
I can't just say how much I love him, but even though he's been established as someone who could be (even though he isn't) as strong as Superman, DC comics doesn't fucking use him for anything important. So yeah, I hate how underrated he is.
The full story is long and complex, but if you like comics and stuff, go check it out. It's really cool. And he's getting a movie if the DCEU doesn't die with Justice League!
So yeah, that's why I kept the name. To remind the world that Captain Marvel exist. And that he's awesome.
Today I started to draw him. I finished Billy, and tomorrow I'll try to go on with the Captain itself. May the Gods be with you.