Airtime is themed. As the name suggests, it's about getting air, and thus all the ramps. It's not themed after a specific real-life setting, but it is oriented around a specific concept and has a fairly consistent texture scheme going on. Blitz was modeled after a real life paintball field, if I recall correctly, so it is themed around a particular setting (though it was executed poorly and I hate the map with a passion, so it's not really supporting the theme cause). Razzle has a bit of a castle theme going on. Barrels was themed after the infamous "crates" map, though that might be a bit of a stretch - same for Midnight 2 being themed after inverting the original midnight (and also sporting a much more consistent texture theme than the first midnight).
Really, though, when you look at it - I don't see much correlation between popularity, quality and theme. I think what's really important is execution. Nothing stops a mapper from making a map that both looks good, has a theme, AND plays well. They just may have to spend a little more time to work it all out. That's why I've been pushing beta releases with public play testing.