To contextualize just how mediocre Mike Tomlin is, consider this:
He has been in Pittsburgh for 10 years. During that time he has had an elite quarterback (top 5, probably top 3) for every year. He has also had a team that is usually ranked in the top 3 to 5 in every preseason analyst's power rankings and is always talked about as a preseason Super Bowl contender due to the caliber of talent on the team. Every year Mike Tomlin has coached, in 10 years, he has had that. The only teams that can compare to that is New England and Green Bay.
In those ten years...Mike Tomlin has won his division 5 times and won 6 playoff games. Bill Belichick has won his division all but one season (nine times) and has 11 playoff victories in the ten years that Mike Tomlin has coached. Mike McCarthy, 7 division titles and 9 playoff wins. Mind you, the Packers are probably less talented all around than Pittsburgh. Even John Harbaugh's, a coach with a much lesser team, has 10 playoff wins in a nine year period.
Considering the talent is at Mike Tomlin's disposal and the fact that he has had a top 3 QB and top 5 roster for his entire career, there is no reason the Steelers should not be deep in the playoffs every season (or at least winning their division, akin to New England). Yet he is not. In fact, only three times in his ten year career, with a Super Bowl caliber roster and a Hall of Fame quarterback, has Mike Tomlin advanced in the playoffs. He has won play off games in three of 10 seasons. That is borderline Marvin Lewis bad.