Yes, stress can lessen your beauty. You can check how it does.

Skin

Stress can alleviate cortisol hormone production and lead to inflammation and breakouts. It also causes dryness, dullness and premature ageing.

Hair 

Stress can cause loss, thinning and breakages of hair.

Nails

Stress can make your nails brittle, weak and peeling.

Seeing all these, you can get upset and be low on confidence. A stressed individual will obviously be less attractive to others. Whereas a relaxed and happy individual will likely feel beautiful and confident.