Is it a good idea to use cocoa butter on sunburn?

No, not a good thing: Keep sunburn cool with moist cool/cold compresses. Avoid alcohol containing products. Break off pieces of aloe vera plant & apply sap to skin. Consider non-steroidal anti-inflammatory meds to reduce pain. Don’t yank off peeling skin or burst blisters. Seek medical care for severe pain, fever, lack of improvement of symptoms within several days or if a large surface area is blistered.