There are wallets doing that already.
Like this one

https://bitcoinpaperwallet.com/bitcoinpaperwallet/generate-wallet.html

You move your mouse until you make the necessary entropy. You can use your keyboard as well with your mouse movements. After that, your private key is generated.

However, I read somewhere that humans are not good sources of entropy.... so maybe that is not a good idea. We problably keep moving the mouse in some crazy pattern.