First, he wasn't saying there was anything wrong with it, he was saying that Jason hasn't been trying to do it.
Aside from that, there is nothing wrong with striving for those results, but you can't "establish the 90's Cowboys", you can only establish something with what you have now. The players are different, the skill sets are different, the defense was built as a 3-4 and not a 4-3, and the simple fact is just because the head coach is a link to that era doesn't automatically make him capable of recreating it. Hopefully there are lessons Garrett learned as a part of those 1990's Cowboys teams that he can draw on now, but he has to figure out what is going to work now, with this team.