No it is not. Capello beating your team with half his stars out is exactly why it isn't. Capello bringing success back to real with a very shit team is exactly why it isnt. Hiring Rafa or Tata is exactly why managers earn what they earn and it exactly why Luis Enrique is leaving.
The richest clubs that get their transfers right are the ones that dominate, totally irrespective of who is manager. Everything else is fleeting and/or an exception that confirms the rule.
Take your ten best managers and put them in 10 poor clubs and they will get the exact same poor results. Pick your 10 worst managers an put them in the ten richest/best clubs and they will get the same good results. There is no evidence in the global history of football to disprove this.
Many say that Pep is horrifically overrated, he still won 2 CLs (among many other titles) Many say that Lucho is 100% clueless, but he still won a treble (among many other titles) If you tell me this was down to the players they had under their disposal then you are merely confirming my basic claim.
The only manager that can claim to have made some kind of discernible difference, is Diego Simeone.