What capabilities do you look for in a PMM? What do you look for in senior PMM?

I think it depends on the organization, and skillset of any existing PMMs (and the PMM leader) on the team today but generally speaking - for a PMM I prioritize strategic thinking, storytelling, and market understanding. For a Sr. PMM, I expect a higher-level of leadership and experience that translate into leading go-to-market initiatives, influencing cross-functional stakeholders, and ideally mentoring others on the team.

For specific skills, the career table here is a good resource on attributes of a PMM vs. a Sr. PMM (and beyond).