In positive characteristic, quotient singularities need not be rational. For an example, see Artin's paper "Wildly ramified Z/2-actions in dimension two" (about which I learned from J\'anos Koll\'ar). 

In characteristic zero, quotient singularities are log terminal, and log terminal singularities are rational. The first fact is a simple exercise on discrepancies. For the second one, a very elementary proof is contained in the book of Koll\'ar and Mori.