Most of the first bots that mutate from a well established original will be worse. That's because mutations are BAD. Evolution is a funny process, going forward and backwards and all around. Don't think of it as a linear process. Think of it like a river. It kind of moves downhill, but it won't take the most direct route, and sometimes it will seem to go backward. Sometimes something slows it and it forms a large lake. Eventually, though, dumb luck will find a useful combination that makes the thing fitter, and it will flow on.
If you want a successful mutation sim, start with a bot that can survive, but just barely. Have it have no bells or whistles, just regular -1 shots and a search and follow gene. Now minimize the program. Step away, and don't touch it.
Come back 3 days later. Is it the same bot? What new behavior does it exhibit? What has the population maxed at?
Now, your first question will be 'is it better?'. Evolution doesn't work like that. Are we 'better' than bacteria? Bacteria outnumber us, can survive in much harsher environments than we can, etc. Whos to say what 'better' is?
Likely, you will just have something new. It might be better, it might not. But whatever it is it will be different.