Rasul (ﷺ) used to say when watching himself in the mirror:

اَللَّهُمَّ حَسَّنْتَ خَلْقِي فَأَحْسِنْ خُلُقِي

Allaahumma hassanta khalqi fa a'hsin khuluqi

O Allah, just as You have made my external features beautiful, make my character beautiful as well.

Reference: Sahih (Albani). Musnad Ahmad: 5099