Is iOS 15.1okay for iPhone 6s

Hello ~Yes ,it is. Take a look here for more information:

iOS 15 - Apple

Update your iPhone, iPad, or iPod touch - Apple Support

~Katana-San~

Hello,

You can choose to stay on iOS 14 for security updates but no feature updates. The latest version of iOS 14 is iOS 14.8.1.

If you want the latest features and security updates then you can go ahead and update to iOS 15.1. So far iOS 15.1 has been pretty stable on my iPhone 7 but if you are concerned about performance then you can just stay on iOS 14 (iOS 14.8.1) and wait a few more months for any issues in iOS 15 to be addressed by Apple. If you do experience any issues with your iPhone that you believe were caused by iOS, you can report them here: apple.com/feedback