Hard core fans are just as suspectible to emotion and unreasonable expectations as casuals. Maybe even more so. And Cowboy fans in particular tend to have unreasonable expectations because they had one of the few dynasties to exist in the NFL and they think it is their right to have another one and any coach that does not establish a dynasty in Dallas is terrible.
Similar things are true about Green Bay fans who are spoiled to have consecutive great QBs and a history of winning Championships.
I'm not saying McCarthy is the best coach in the league and he's not top 5. I'm not even saying he's definitely the right HC for the Saints. But, he's a very good coach with a very high win percentage over his career and lots of success in the playoffs including one Super Bowl. But, none of that is going to matter to Cowboy fans, especially the older and more hardcore ones, because that success did not happen in Dallas and to them it's either win several Super Bowls in a short period of time or you aren't good enough to coach the Cowboys. Some of our fans have become like that, although without any real justification given our history.