How to get a brown/burned texture on cornbread?
Most likely you just need to turn up the heat a bit. Moving the rack higher might help too.

It's a little hard to say without actually measuring your oven, but that kind of thing can happen if your oven's just a bit cooler than it claims to be. So it'll take a little longer than the recipe says to bake, but the lower temperature means it won't brown as nicely. Turning up the heat will get more browning done by the time the center's done.