Would a teaspoon be a good way to measure the capacity of a milk carton? explain

Yup, because it's totally efficient to scoop out teaspoon by teaspoon of milk.

NOT

Many containers and cartons of volume are measured in milliliters; a teaspoon is traditionally 5mL, so as a unit to measure capacity: yes, they are good.
However, in a 2 liter bottle (common size) there are 2000/5 = 400 teaspoons. While this is easy enough to write on the label - indeed, soft drink cans are often 355 ml, so there is no real difference bar that of unit - it is extremely difficult to measure out 400 teaspoons accurately. As a way to fill that capacity (i.e. "measure it out"): no, they are terrible. Hope this Helps! :D