You're defining "classist" to mean "anything that is about class", but that's not really right. A better definition of "classist" is "actions that enforce class

You're defining "classist" to mean "anything that is about class", but that's not really right. A better definition of "classist" is "actions that enforce class systems", and by that definition being against fast fashion is not inherently classist. However, one can advocate against fast fashion in a classist way.