Yes, angelfish are happy alone. This is especially true if the tank is small. Most angelfish are territorial, so they don’t mind being alone in a tank as long as it has lots of foliage and decorations to interact with.
However, angelfish are naturally social animals and boredom can negatively impact their health. Angelfish thrive best in small schools of 5-6 fish. Make sure each fish gets at least 10 gallons of water.