They don’t get better their stats are fixed at outset. As you progress as a Manager and reach higher levels you need to upgrade your staff (specifically CD, but also TD). If you don’t ensure your CD level keeps pace with your Manager level it will negatively impact on your new season car design.
Edit: you need to replace both your TD and CD. Your TD’s level is too low for your manager level and your CD has the worst strengths and weaknesses you can get. Cooling has no positive impact on your car and having Acceleration as a weakness is really bad for your car. If they were the other way round, i.e Acceleration as your strength and Cooling as your weakness you would have a good CD for your current level.