I must ask back: in what context? Because PMM can flex its priorities and alignment towards Product vs. Sales vs. Customer Success (or even Marketing demand gen/comms, I go back do ensuring that whatever KPI is set supports a top 3 corporate objective that the C-Suite cares about.
If you forced me to point to a specific KPI, I'd pick #leads or $pipeline generated. You see this especially at smaller, early stage companies where the Marketing team is just getting built out and the team is trying to distill down to 1 KPI a few team members can get behind -- you can map the success of a new product launch... or the ability to feed sales... to top-of-funnel interest.
However, I'd work backwards and instead set the KPIs to the ultimate objective vs. something in the interim. E.g., If you set the KPI as X% activation or adoption you would need signups (leads) to get there anyway. If you set the KPI as X$ revenue you would need pipe to get there anyway, etc. And this isn't a knock on the demandgen or customer marketing teams. Leads and pipe are important metrics, just not for PMM.
My runner-up unimportant KPIs for PMM: # views of a particular piece of asset, followed by aggregate NPS (unless you have a very mature and built out team that can consistently cover all personas and a ton of ground).