Answer: The sense of taste and smell are related to each other, hence Cosmo's sense of taste is altered because of his stuffy nose.
Step-by-step explanation:
It is common to feel like you can't taste food when sick, because the sense of smell and taste are related to each other.
Smell essentially influences our ability to taste food, which is why distinguishing flavors with a cold (or stuffy nose) is hard sometimes.
When drinking coffee, the aroma of the coffee travels through the nasal area. There, the hair cells carry the odour related information to the brain. But Cosmo's nose is stuffed to the point that he can't smell anything, so due to the mucus in his nose, he is unable to smell the odour, hence his brain is not able to tell him anything about the flavour of the coffee. As a result, he thinks it has no taste.