1 You are not familiar with it
You might be used to looking yourself in your bathrooms, or dressing or bedroom mirror rather than looking yourself in the car's side mirror or front shield mirror in sunlight. You may find yourself unattractive as you are familiar with seeing your face in artificial lighting rather than in sunlight.
2 Natural Sunlight makes imperfections prominent
Your skin flaws become pretty much visible in direct natural sunlight.
3 Your face appears less symmetrical
Our faces may seem to be unattractive in sunlight as it might cause to appear some blemishes on your skin. Your face may look slightly unsymmetrical as there are fewer chances that you get an equal amount of sunlight on both sides of your face. See Why do I look ugly in some Mirrors?
4 Acne and scars are highlighted in sunlight
Whatever marks, spots, and discoloration you have on your skin will be highlighted in sunlight, making you look ugly.
5 Sweat can affect how people look
The looks of a person can be changed if sweat is released over your face. Even if it is a thin layer, it may change light properties and result in changing your looks.
6 Bad weather can be one of the factors
The weather is a significant variable in determining your looks when you are looking at yourself in sunlight. Bad weather can affect the health of your skin and make it vulnerable. At that point, if you look yourself in the mirror, you might find yourself ugly.
7 You just woke up
If you recently woke up and did not even take a shower, you might find yourself a little unattractive, especially when you look yourself in the sunlight. See also Why do I look Ugly in Videos?