It's not a hat, it's a karakasa obake, a japanese monster. It looks like an umbrella with one eye, a long tongue and a single foot:
https://en.wikipedia.org/wiki/Kasa-obake
It's a wreid monster at first sight but they're funny imho.
Poinsettia is maybe my favorite 5* from this event. Silene is nice but Poinsettia is more cute with her monster umbrella. Too sad she lost it with her evolved form.