Myself, I think he's not. He's had the benefit of having a HOF QB his entire career, with Ben he's never had a losing season. When Ben retires, the losing seasons will pile up fast, he's nothing, he's been carried by Ben.
Then who is better? Name an active coach that you'd put above Tomlin that hasn't had a franchise QB for most of their tenure.
Belicheck? He has had Brady for almost his entire career, and certainly his entire successful career. His record before Brady is 36-60.
McCarthy has had Brett Favre and Aaron Rodgers for his entire career. How'd he do last year without Rodgers? His team went 3-6. His Packers consistently rank near the bottom of the league in both rushing offense and virtually every defensive category.
Andy Reid? Had 6-time Pro Bowler McNabb (85 lifetime Passer Rating)for most of his career. He's no Ben, but he was a top tier qb for nearly a decade. Has a sub .500 Playoff record.
John Harbaugh? He's had Flacco his entire career. Again, Flacco isn't Ben but he is statistically very similar to McNabb and a top tier QB of his time. He's also missed the playoffs 4 of the last 5 seasons, while Tomlin's made the playoffs 4 out of the last 5.
Pete Carroll? 47-49 before 4-time Pro Bowler Russell Wilson. Wilson who will be a shoe-in for the HOF if he keeps playing the way he has.
Sean Payton? First ballot HOFer Drew Brees during his entire career, and still has almost as many losing seasons as winning season (4 vs 6). His defense is consistently one of the worst in the NFL almost every season (28th, 32nd, 31st, and 10th in points allowed in the last 4 years).
Ron Rivera? Mike Zimmer? How far down the list would you place Tomlin?
BTW Tomlin is 10-8 in games without Roethlisberger. Not amazing, but still a winning coach with a back-up QB.
I'm not saying that he's better than all the coaches listed, but Tomlin's accomplished more than all but 1 or 2 active NFL coaches.