You're correct that he is a showman, but any showman needs to deliver periodically. Otherwise he becomes uninteresting or even foolish, depending on how over-the-top the behavior is (and Deion is pretty over-the-top).
The reason for the meteoric rise in the interest of Colorado football isn't just because Deion ran his mouth all offseason - it's because he ran his mouth all offseason and for the early part of the season actually delivered. He definitely attracted attention this offseason, but for the most part nobody really took him seriously. Beating TCU as a 3 TD underdog, however overrated they might be, and then following that up with a dominant victory in a roughly even game actually got people to start taking his team's performance on the field seriously (e.g., noticing that his son and Travis Hunter aren't just names; they can produce on the field). When you combine his showmanship with actually winning games, you end up with the insane buzz that was created.
Nobody expects him to beat or even compete with good teams, hence he can get his teeth kicked in by Oregon and USC and be OK. However he really starts to become far less interesting, and the whole thing starts to become overplayed, if he starts losing games that are seen to be even or where they're favored. If he does all that stuff but then can't beat Stanford, people move on to something more interesting.