It is currently already the case that the creature with the maximum distance from the ground (measured from the lowest joint) is given the highest fitness score, I'm not choosing the highest center of gravity.
Thinking about it though, I should probably change it to optimize some sort of a weighted average between the lowest and highest joint height, because currently, if there's two creatures - one doesn't move at all and the other one extends itself upwards but without actually leaving the ground - the algorithm gives both of them a score of 0 because they both effectively didn't jump at all. However, it makes a lot of sense to give creature no.2 a higher fitness score because it was a lot closer to jumping than creature no. 1.
I'm going to play around with this for the next update to see what I can do. Thanks for pointing it out!