Hello Greg,
It sounds like you're wanting to change the video quality for Apple Fitness+ only, while keeping the quality the same for the Apple TV. Being able to manage the video quality is important for any device, and you can adjust the video quality for the Apple TV using the steps in the Adjust video settings of the following article.
Adjust video and audio settings on Apple TV
in reference to adjust the settings specifically for Apple Fitness+ it would be recommended to provide feedback to Apple. They do appreciate feedback for all features and devices. You can provide feedback via the Product Feedback website.
Thank you for reaching out to Apple Support Communities, have a great week.