Coaches skirt by all the time on the quality of their roster; 85% of winning in any sport is the talent available. You can determine how good a HC is in 3 key ways:
1. How prepared his teams are for their opponents.
Do they come out flat? Do they ever look overwhelmed or outmatched? Do they consistently lose games to lesser competition?
The answers to all of these are not in favor of Tomlin. His teams regularly look unprepared, flat, overwhelmed, and/or outmatched. They consistently lose to weaker opponents, and have embarrassed themselves frequently in big games.
2. Game management.
Whether it be in-game adjustments, situational decisions, or timeout/ clock management, a good coach should have a mastery of this part of the game.
Situations that require a mastery of situational football do not confound good coaches, but they do Mike Tomlin. Almost every game we can point to decisions made that confound even the broadcasters.
3. The most important indicator: success in spite of talent. This indicates a deep knowledge of the game, an ability to adjust, and a high level of scheming. It doesn't necessarily mean winning championships, but it does mean their team is routinely punching above its weight.
Again, this is not in Tomlin's favor. Tomlin's supporters often point to the year without Ben where they went 8-8. On the surface, it does look like a decent coaching job, but any further analysis reveals it wasn't anything special. The Steelers that year had possibly the easiest schedule of all time. They had just one win over a team with a winning record, the 9-7 Rams. They scrapped with the bottom of the league and emerged with a .500 record. Not that impressive, and all because they couldn't adjust to not having Ben. They still had a good OL. They still had very good skill position players. Yet they could not make any of it work on offense. They finished second-to-last in passing and fourth-to-last in rushing - again, against the league’s easiest schedule. This tells us Tomlin leans heavily on the individual talent of his players, does little to no complex scheming, and has little ability to adjust.
So no, Mike Tomlin is not a good coach. He's probably not even a decent coach. It's likely he's a bad coach who's been extremely lucky to have a HOF QB and a HOF GM propping him up, and a whole bunch of people willing to make excuses for him.