This is something that's popular to mention, but not all that true. Most of what we see in Microsoft now, started under Ballmer's leadership. UWP, Windows on Arm, Surface, HoloLens, etc. More importantly Xbox as a unified experience. That last, however, was problematic as the head of Xbox at the time, Don Mattrick was still of the opinion that Xbox should be a separate OS from Windows. However, compromises were made. The UI was largely influenced by Windows Metro design language. And work had started for UWP (in a slightly different form from UWP as it is now) app. integration.
What Ballmer was, however, was representing the old guard of Microsoft. He wasn't under the control of the investors, and he'd been at the helm when technically good products had launch, but received a lot of negative publicity. Vista for instance is a solid OS and upgrade over Windows XP. But bad drivers by certain companies completely ruined its image (similar to Windows XP at launch). Windows 7, which was basically just Vista with a new skin launch to critical acclaim, although it was basically just Vista. At a time when iOS (highly profitable) and Android (marginally profitable) were taking market share, albeit not sales (at the time), away from Windows, that all combined to make him less popular with the investors.
If anything, Ballmer was a more conservative businessman than a risk taking businessman. As Dsoup mentioned previously, Investors sometimes like big risk if it can result in big rewards. Explosive growth can dramatically drive share prices up, while conservative growth doesn't. Ballmer was the antithesis of what the investors from financial instituations on the Board of Directors wanted (high risk, high reward) but favored by members of the board who wanted stability and conservative growth like Bill Gates. And you can't blame him for being that way, Microsoft's largest revenue generators, small business and corporations, value stability over the volatility that can come with high risk/high reward strategies.
It should be noted that it was those same financial investors that started the push to get Ballmer out of office, and not the traditional people you'd see on the Board of Directors (like at say Apple or Google).
Regards,
SB