I would certainly argue that appeasement was a mistake. I suppose you could argue that it was a mistake before WWII and not a mistake in that war. However, it seems hard to say that appeasement was anything but a mistake.
Appeasement was the process in which the British and the French, in particular, allowed Hitler to violate the Treat of Versailles and, eventually, to take over other countries, without resisting him. They allowed Hitler to do so because they did not want a war. They hoped that Hitler would soon be satisfied and would then stop his aggressive behavior.
Appeasement was a mistake because it did not prevent war. Instead, it only postponed the war, which was actually a bad thing. Postponing the war was a bad thing because all it did was to give Hitler time to increase his power. When Hitler started violating the Treaty of Versailles, Germany was still rather weak. The French and the British could easily have invaded Germany at that point and defeated Hitler. If they had done so, the Nazi government would probably have fallen and the world might have been spared WWII and the Holocaust. Instead, the allies appeased Hitler and gave him time in which Germany could rearm and become a real power. Once the Allies decided that they actually had to fight, the German military was extremely strong and was able to rampage through Western Europe during the Blitzkrieg. This could have been avoided if the Allies had not appeased Hitler.
There is nothing wrong with avoiding war. However, when you fail to actually avoid war and, instead, you only postpone it until your enemy is stronger than you, you have made a terrible mistake.