"Data governance maturity" could be taken as a part of the more general notion of institutional maturity. There are various models of institutional maturity, discussed in the field of public administration, such as this:
Thynne, I. (2012). Institutional maturity and challenges for integrity bodies. Policy Studies, 33(1) [Special Issue: Understanding Integrity in Public Administration], 37-47. doi:10.1080/01442872.2011.601202
Recently, I was reviewing this literature (for a policy research project). Some of the literature also draws upon the work related to the SEI-CMM model and its variations. On the whole, I find that a three-stage model of maturity makes sense:
Stage 1. Mere presence of policies and procedures which are necessary for responsible conduct towards fulfilling stakeholders' expectations
Stage 2. Effective functioning of those policies and procedures, including ongoing review and learning, leading to periodic revision and development of those policies and procedures.
Stage 3. An entrenched culture of excellence, which ensures that all institutional actions and decisions are aligned with the broader ideals of quality and integrity in serving the stakeholders as well as the broader society.
In this model, as we go towards higher levels of maturity, it becomes more difficult to measure it in precise terms. I think, Stage 3 maturity can only be assessed approximately through a combination of objective and subjective criteria (including some form of independent peer review).