This meter displays the playable Yoshi's health. Each pedal on the flower is one point of health for the Yoshi, and once all of the pedals are plucked, if Yoshi gets hit it will lose a life and be taken to Bowser's Castle. If the current Yoshi is squished or falls into an abyss, it will lose all of its pedals and lose its life.
As the Smile Meter depletes, it becomes a sadder flower. The smile on the flower becomes a discomforted frown as it nears its end, having no pedals to protect the Yoshi. If a Heart Fruit is eaten, then the Smile Meter grows an ecstatic, happy smile for a limited amount of time.
Yoshis can primarily replenish the Smile Meter by way of eating fruits. Depending on the color of the Yoshi, different fruits will replenish more pedals for the Yoshi. Grabbing Special Hearts or Heart Fruits can also heal a Yoshi's Smile Meter.