Selfies are usually taken from a very close distance. As the angle and the distance change, the picture seems very unusual compared to the other pictures you took before. See why do people take selfies.
Phones have wide angle lenses, and as a result, they might distort the looks of closer features and make them seem different than they truly are. For example, your nose might look bigger and your face might look narrower. See why do you look ugly in sunlight.
Because selfies are usually taken from a short distance, they make certain parts of the face look larger. The closer parts to the camera might seem much bigger than they truly are.
Most humans, with the exception of a very small percentage, don't have a perfectly symmetrical face. This is why our reversed images in mirrors appear different than our non-reversed images in pictures. See why you look ugly in mirrors.
In many phones, the front camera is not as good as the phone's rear camera and this is why pictures taken with this camera might seem less attractive. See also why people make duck faces in selfies.
According to a study, people rated the pictures taken by themselves less attractive than the pictures taken by others for them. The only exception were narcissists who preferred the pictures they took for themselves.
Most people have certain angles that they are comfortable with, for they are used to their looks when looking at themselves from those angles. As selfies might force people to change the angles, the pictures they take might look weird.
Selfies are usually taken on the go, and as a result, people might not have the chance to adjust the lighting conditions. The absence of proper lighting can result in bad looking pictures. See why you look bad in videos.
When you look in the mirror you see a reversed image of your face. When you take a selfie you see the normal non-reversed image of your face which you are not used to seeing. This is why it might seem odd and weird to you. See why you look bad in photos.
If you haven't taken many selfies, your pictures will probably not seem familiar to you. According to psychologists, familiarity increases attraction and this is why people might find their selfies weird-looking in the first few times.