A selfie should not be used as a headshot. They lack the skill of a professionally taken headshot, and will seem amateurish in comparison. A headshot can be taken on a phone, but always ask someone for assistance.
It’s harder to connect with the camera when taking a selfie. As you’re focusing on two things at once, the pose and the photo, sacrifices have to be made. This either leads to a shoddy photo, or a model who doesn’t connect with the camera in the same way.
Ideally, a headshot will be taken by a professional. Remember that it’s being used to make you money. It’s better to pay more upfront, as you’re more likely to see the returns.