Well, it might not be serious if it was only a little bit. But if your cat drank a lot of it, you should probably bring it to a vet or you can call poison control for helpful information. Be careful about what you let your cat get into, certain substances can be dangerous.
Methanol (wood alcohol) is toxic in small amounts; isopropyl alcohol is little better but not as toxic. Isopropyl alcohol and its metabolite, acetone, act as central nervous system (CNS) depressants. Around 15 g of isopropyl alcohol can have a toxic effect on a 70 kg human if left untreated. However, it is not nearly as toxic as methanol or ethylene glycol. Isopropyl alcohol does not cause an anion gap acidosis (in which as lowered blood serum pH causes depletion of bicarbonate anion) unlike ethanol and methanol. Isopropyl alcohol does however, produce an osmolal gap between the calculated and measured osmolalities of serum, as do the other alcohols. Overdoses may cause a fruity odor on the breath as a result of its metabolism to acetone, which is further metabolized to produce the nutrients acetate and glucose. Treatment is usually sodium bicarbonate, best administered by a vet.