Poinsettias are acid loving plants so yes coffee grounds are wonderful for putting in the soil of your Poinsettias.
I use coffee grounds every once in awhile to add some acid to the soil of my Poinsettias and it helps them a lot.
Just do not put too much coffee grounds in the soil of your Poinsettias otherwise the soil can become too acidic and the Poinsettias can die as a result.
Coffee grounds are also good for other plants such as tomatoes, Avocados, Blueberries, azaleas, evergreens and camellias as well as many other plants.