Adding a mulch of cedar needles around tomato plants can be good if you need to shade the soil, but not so good if you are depending on rainfall to help keep the roots moist. Cedar will repel water. If you are thinking the cedar will help make the soil more acid, that is true but it takes a long time (years) to change the acidity of the soil this way and tomatoes do not require an extra acid soil the way blueberries, rhododendrons, or azaleas do. I would use a wood chip mulch around the tomato plants instead. Keep growing, Marianne Binetti