This is a web extension that only displays grades over 80% under the 'recent feedback' panel of Canvas.
Chrome Web Store: https://goo.gl/41gCiE
Firefox Addons: https://goo.gl/7zUIO6
An alternative web extension is 'Canvas Hide Recent Feedback' which simply hides all grades.
Chrome Web Store: https://goo.gl/xjwt5Y
Firefox Addons: https://goo.gl/AcYbnk