You can use your fingers to apply your foundation as the warmth of your hands works to help you blend the product in better. However, you will have to ensure that your fingers are properly cleansed and sanitized before touching your face.
In addition, you can have streaky marks with your fingers that you may need to blend out. For a professional look, we would recommend using a brush.
The bottom line is that you should use whatever works best for you. If you find that using your fingertips to apply your foundation looks great, then do it!