By default, top sorts by PID. Just tap "o" on the (virtual) keyboard and enter "cpu" to sort by CPU usage. (Add the "+" sign if you want the list to be sorted increasingly. I don't recommend this when using the on-device terminal as, on iOS5+, there will be too many daemons / processes and the top CPU users won't be displayed. When accessed thru SSH, however, you can enlarge the terminal window.)
Another very useful command is "command", which sorts by the command name if you don't want to manually search for a particular process. (Here, adding a "+" on even the built-in MobileTerminal app can indeed be meaningful.) Also, "rsize" sorts by the memory usage - this is also very important.
Just remember to always tap "q" before exiting; otherwise, top will remain active even after exiting MobileTerminal, resulting in some excess CPU usage. For example, this happened to me yesterday, running my iPhone 5's CPU at 100% always and quickly chewing thru my battery: see the attached screenshot.