it sure is! Just go to the opponent manager menu and click the driver's ratings. Just be careful you don't want to make one driver to weak or to strong you can't beat them.
I believe he is referring to the AI Oponnent Strength (The % in the race menu), from week to week.
Within the game itself, you cannont. However, I do remember a program that allowed you to change it, but I don't have it anymore\don't remember what it is called. I think "Season Editor" or something, but I don't have a link.
If you start your season using the auto ai strength you can adjust the ai strength for each track. There is a file in your player folder called playrecs. Open it and each track will be listed with the auto ai strength and you can change it to whatever percent you want for each track.