Why are you surprised? Ballmer was an idiot, just check his onstage monkey dance on youtube. He had zero technical knowledge and vision, and his pet project was a computer screen on a kitchen table (the original Surface).
But he was probably good in one thing - he knew how to climb the corporate laddar, and how to surround himself with loyal idiots to exchange favors with.
The company took a nosedive under his direction, basically everything from his era was wrong, Windows 8 and Windows Mobile being the most glaring examples.
Now that he is gone, his loyal henchmen steer the wheel. The results correspond to their abilities.