Let's say the number of cupcakes that JoJo decorates is J.
We know that Darcy decorates 8 cupcakes, which is 2 more than 1/3 the number of cupcakes that JoJo decorates. This can be written as 8 = (1/3)J + 2.
Subtracting 2 from both sides, we get 6 = (1/3)J.
Multiplying both sides by 3 to get rid of the fraction, we get 18 = J.
Thus, JoJo decorates 18 cupcakes. Answer: \boxed{18}.