Mark's a very average bassist in terms of technique. However, I could care less about "chops" - creativity and cleverness are much more important. Unfortunately, he's also not very good at that. Most of the time, anyway.
But the multiple bass tracks on "Always" are incredible. Actually, he did some interesting stuff all over the Untitled record. And California seemed like a step forward in many ways too.
Really, the best bass skill Mark has is knowing how to play something simple, but just different enough from what Tom or Matt is playing. That's the talent. And he deserves props for that.
And at least he's not an annoying bassist. There's nothing worse than a tacky bass line that just doesn't fit the song and seems needlessly complex or otherwise awkward. He steers clear of that.