I've heard of Dido only once about 10 years ago because a coworker liked her music. I haven't listened to any of her songs that I'm aware of.
But just to play devil's advocate and provide some perspective for you...
21 million copies sold of her debut album
There are 6.8 billion people in the world and she sold only 21 million albums. Only 0.3 percent of the population has bought her stuff. She doesn't seem to be that popular.
MTV caters to a very narrow subset of musical genres and artists. There's far more in the musical landscape than what they represent.
I have no idea what that is.
That doesn't mean much. The Grammys are the opposite of the MTV music awards as there are Grammys for all types of music, even polka. And nominated doesn't mean they won. Many artists have been nominated.
#98 best selling of the 21st century
#98 world wide counting all countries music sales across all genres?
Isn't Eminem a rapper/hip-hop artist? There are a lot of people who don't listen to that. For example, I listen to jazz and classical. I'd be hard pressed to name more than a couple of artists outside of those two genres. For example, I couldn't name any other rappers except for Ice-T (because he's in Law and Order) and Timbaland (because he was on slashdot having been caught as a plagiarist).
music featured in a big movie
I've never seen a movie where they display the name of the song and artist currently playing. If you didn't know of the song beforehand, you won't magically know the artist just because it was playing in a movie.
song the opening theme of a US TV show
There are thousands of US TV shows. Not everyone watches the same ones.
Is that a measure of success? I can't think of any haircuts with names other than a crew cut.
This is only of note to people who have heard of her and want to buy her product. The same can be said of many artists that most of us have never heard of. Lana Lane has had several sold-out world tours. I doubt most people have heard of her.