I certainly think it's too much for him, even though he's a Top 5 NFL QB.
But in his defense, he does has a playoff win every year in the NFL, and he does have an above average arm. Great deep ball thrower, and he's a relatively low risk high reward type of QB. He's a talented game manager type of QB but typically doesn't throw very bad balls. Most times he's not going to lose a game for you, but he certainly has the skills to take you down the field and win one for you.
He already has 9 playoff wins in only 5 years, which puts him tied for 9th all time and he's got some great weapons on offense. Now I think sports players get paid too much already so I think 120.6 million is too much for anyone and if anyone was going to be paid that amount, I don't think it should be Flacco. But I can understand why they did it. Serviceable OL, good RB, good WR, pretty good DL. They needed to lock up their QB so they can retool their defense in the coming years with the draft. They will be set on offense for years, so if they can get younger on defense, they are going to be around an AFC contender for a while.