Rather than a KPI, the things I see teams miss most often is looking at output rather than impact. Before setting KPIs, it is key to establish what you want to achieve and why at an organizational level. After companies set their north star and goals, that should flow into department priorities, team priorities, and individual priorities.
Product marketers can then use an output to impact framework to plan their work. Start with the impact on the organizational level (ARR, new subscribers, churn, ACV, pricing tier adoption, CAC, win rate, LTV, etc). Then examine how you could help the organization achieve those impacts through your work. Map how your work will ladder up to the impact and identify what the leading indicators will be. So for example, if the company has an ARR target, you can impact by identifying and helping get a feature prioritized that is resulting in closed lost. Through a GTM campaign that drives awareness, you can use a combination of campaign and conversion metrics and win loss data to measure your impact -- closed lost for X reason decreased by Y%, increasing conversion by Z%, ultimately resulting in _ net new ARR.