Karl-Heinz Däke, from the Federation of Taxpayers told news agency AFP: "I can't believe the cheek," referring to the rare €2.8 million ($3.8 million) campaign that appeared in every major daily newspaper.
"Thank you, dear citizens. You have made Germany the country that has best overcome the global economic crisis," Merkel wrote in the large ad from the government, not her conservative Christian Democratic Union (CDU) party.
"The world looks at our country and is talking of a miracle. I do not believe in miracles, but I do believe in the people of this country, in their ideas, in their good sense and in their engagement."
But she also took some of the credit for getting Europe's biggest economy through its worst post-war recession in 2009, saying government measures had kept a lid on unemployment and cut financial slack for families and firms.
Since winning a second term in September 2009, Merkel's popularity ratings have fallen ahead of a crunch year when six of Germany's sixteen states hold elections.
At a conference of her conservative party on Monday, Merkel said that voters would soon warm to her government once the benefits of her "contentious" policies on energy, tax and healthcare became clear.
Germany, the world's second top exporter after China, was hit harder than most by the global recession, as demand for its goods dried up.
However, it has rebounded impressively and the forecast from Berlin is of 3.4 percent growth this year.