It varies from pastor to pastor, there is nothing official so far as I am aware. I have seen parishes where they wear suit/tie or nice dresses, others (much rarer, thankfully) where they dress “just like folk” in t-shirts and jeans – in most they dress like most of the laity, which is to say reasonably nice and professional if not very formal, e.g., dress shirts and nice slacks. It really depends on the sensibilities and initiatives of the pastor. I know of at least one parish where the pastor, in an effort to induce the laity to take Mass more seriously and dress more carefully for it, required all his readers and EMHCs to dress formally (e.g., coat and tie for the men) or be dismissed from service, for instance.
Generally, yes, I would say shorts are extremely inappropriate for Mass, but they are inappropriate in a moral or devotional sense, not necessarily a legal/canonical one.