BERLIN—Hans-Werner Sinn, head of the Munich-based Ifo economic institute and arguably Germany's most prominent economist, is also among the euro-zone crisis management's most vocal critics. He has lambasted the European Central Bank and the German government since the crisis broke out in 2010. As the ECB's governing council meets Thursday, he tells the WSJ the bank is acting outside its mandate at considerable risk to German taxpayers. With yields on government bonds falling ever lower, he adds, no euro-zone government apart from Ireland's has undertaken unpopular but necessary structural overhauls. Labor costs in most...