According to serrebi clefairy has a 5% chance to have the upbeat mark in the ongoing event. Does this stack with title power 3?
The Upbeat Mark is a Personality mark that confers the "the Chipper" Title when equipped on a Pokemon.
Normally, there is roughly a 1/100 chance to find a Pokemon with the Upbeat Mark in the wild. Make use of the event, and further increase your chances with sandwiches to catch yourself some rare Pokemon with extra added flair with titles!
Seems like it does stack. Sandwiches stack on other Mass Outbreak things, so the mark odds should also stack. Source