Islam is a complete religion. It teaches us the etiquette for each and every aspect of Life. We learn manners for not just what relates to us but also to others. Reciting the dua for someone who is wearing a new garment is among those etiquettes.
Prophet Muhammad (ﷺ) appreciated and complimented everyone around him. So much that everyone would feel loved in his presence. His sunnah is to be well-mannered and the best in Ikhlaq (pleasant behavior). We learn from reliable sources of Hadiths how when he saw someone wear a new garment, and he would pray for and upon them.
We see his habit of appreciation modeled in one anecdote reported in Sahih al-Bukhari 5845. Umme Khalid bint Khalid (رَضِيَ ٱللَّٰهُ عَنْهَا) narrated that Allah's Messenger (ﷺ) called upon her, dressed her with a black khamisa with his own hands, and prayed, "May you live so long that you will wear out many garments." In a hadith recorded in Sunan Abi Dawud 4020, we learn the dua we should invoke on someone wearing a new garment. Umrah bin Aun (رَضِيَ ٱللَّٰهُ عَنْهُ) narrated that Abu Nadrah said: When any of the Companions of the Prophet (ﷺ) put on a new garment, the Prophet (ﷺ) blessed them with this prayer, "May you wear it out and may Allah (سُبْحَانَهُ وَتَعَالَى) give you another in its place."
Home & Family Duas
Tublee wa yukhliful-laahu ta'aalaa.
May Allah replace it when it is worn out.
When you see someone wearing new clothes, compliment them and recite the aforementioned Duas. To replicate the sunnah, observe and appreciate the garment and then offer dua for them.
Saying a dua over someone wearing a new garment fills the other with good feelings for you. An exemplary Muslim has a beautiful Ikhlaq and is kind to everyone around him. Supplications for seemingly mundane matters instill in us the remembrance and faith in Allah (سُبْحَانَهُ وَتَعَالَى).