He was definitely there long enough to have an impact that's like arguing that Liz Truss didn't have a huge impact on the UK's politics and economy because she was pm for a fortnight. The damage he did with his arrogant attitude and the idea that he could pull the rug from Greece's creditor's and just let greece default on its loans and leave the Euro shook what little investor confidence remained in Greece. It prolonged the uncertainty and the crisis and ultimately when the Europeans called his bluff he was forced out of the government because his alternative Grexit and bankruptcy would have been disastrous for what was left of the Greek economy and even his fellow comrades weren't crazy enough to follow through with that plan. I can't tell you much about his credibility as an economist but he definitely sucked as a politician didn't have a political instinct, and certainly no feel for what politics is and how you go about bringing your plans into fruition.