Frogs are cold-blooded, which is how they get away with having only one ventricle. Their cells use less energy because they do not have to maintain a constant body temperature. Therefore, the rate of cellular respiration in a frog's cells is reduced considerably. Since they don't need as much oxygen for the creation of as much ATP as do warm-blooded animals, the single ventricle pumping out blood that is mixed (oxygenated with deoxygenated) is gives adequate oxygen for the frog's biological functions. Warm-blood animals (mammals, birds) must have that division between oxygenated blood and deoxygenated blood, so they have a separate ventricle for pumping out only oxygenated blood to the body and the other ventricle for pumping deoxygenated blood to the lungs for transfer and pick-up of more oxygen. This results in a far superior oxygen transport system in the blood, which allows for the much higher metabolic rates that warm-blooded creatures need.
Frogs are cold-blooded; that's the basic reason they don't need as much oxygen in their blood, which is why they only need one ventricle and the blood can mix and still keep the frog alive.
The presence of a single ventricle means that the oxygenated blood is mixed with deoxygenated blood and this means that the system that deals with the transport of oxygen is not performing and it does not allow higher metabolic rates.
You need to know the fact that the frogs' cells do not need higher amounts of energy since they do not have to keep a constant temperature of the body, hence these cells do not need higher amounts of the end product ATP since the metabolic rates are not so high.
Hence, the presence of only a single ventricle is not a problem in frogs because the temperature of its body does not have to be kept constant.