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This article will help you with understanding more about pinching plants: https://www.gardeningknowhow.com/garden-how-to/info/pinching-plant.htm
Garden mums are not as hardy as nurseries would like people to believe. They have a hard time surviving the winter and you can expect about a 50/50 chance of them surviving regardless of what you do. You trimmed them correctly. It was the winter cold that killed them.