You wouldn't eat 13 candy canes, so why would you drink the equivalent?
Because Starbucks is being naughty and is trying to fill your stomach with way too many calories and over 54 grams of sugar with their seasonal peppermint coffee drink.
A Grande (16oz) version of the peppermint mocha coffee (made with 2% milk) contains 410 calories, 15 grams of fat and 63 grams of carbohydrates. Yikes. That's almost 3 times as many calories as one of their chewy chocolate chunk flour-less cookies.
If you want a treat, go with a mint tea and one of the cookies, or better yet, a mint tea and a BioTrust protein bar. If you must have one of these high-calorie coffees, have it after your workout when your muscles will suck up the sugar. You can also limit the damage by eliminating the 'pumps of added syrup'. Each one contains 20 calories of sugar (5 grams, equivalent to nearly one teaspoon).
Another way is to eliminate the whipped cream (that cuts 80 calories and 7 grams of fat). Next, go with an 8oz version rather than the 16 oz version. Using those tricks, you've now turned that 410 calorie coffee into a short (8oz) peppermint mocha coffee with just 160 calories and "only" 26 grams of sugar. That's still too much sugar, but if you drink just one per week as a treat, that's fine. Personally, I'd rather have the cookie.

Source: ETR