Let me start by saying this: I love pumpkin-flavored anything. And I love frozen yogurt. I will happily scarf down any Red Mango or Pinkberry put in front of me. That said, I'm heartily disappointed in both the chains' seasonal pumpkin offerings.
Red Mango's "Pumpkin Spice" flavor tastes so un-pumpkiny that one blind taster couldn't tell it apart from their plain frozen yogurt; the graham cracker crumbs don't add much to the experience. Pinkberry's is similarly lacking in pumpkin flavor. It was served to us with graham crackers and whipped topping, but the only remotely autumnal element was the sprinkle of cinnamon on top. Tangy and fresh, it essentially tasted like Pinkberry's standard yogurt, if a bit less tart. (Some tasters thought it was an orange flavor; others, no different from the original.)
All the more disappointing because I've had such excellent pumpkin elsewhere. At Culture in Palo Alto, California, where the yogurt is made with milk from the highly acclaimed organic Straus Family Creamery, the fall pumpkin flavor tastes like exactly that, a pumpkin pie filling whipped into creamy, cold yogurt form. (Amp up the pumpkin by topping it with pumpkin seed brittle, or even a pumpkin crisp, available to scoop on top.) And Froyo World in New Haven had an excellent version a few weeks back, creamy and not too sweet, tasting equally of pumpkin and wintry spices.
So yogurt chains—take a lesson from the little guys. It's possible to make great pumpkin frozen yogurt. Just make it taste like pumpkin before you slap a name on it.