Porter de beaux vêtements peut vraiment redonner le sourire à quelqu'un.
The verb pouvoir is third person singular (peut) because the subject is the phrase porter de beaux vêtements. Incidentally, that's just like in English:
Wearing beautiful clothes makes me happy.
The verb "to make" is also in third person singular (makes) for all the same reasons as in French.
Notice that if we make "beautiful clothes" to the subject, the verb adapts to that and now is 3rd person plural:
Beautiful clothes make me happy.