When I think of the term "opinionated" as it applies to software platforms, I think of things that have intended purposes in mind and optimize for the ease of those use cases, potentially to the detriment of people doing other things. It also suggests a "one way to do it" approach.
If that is the definition, I don't think Postgres fits. Postgres tries pretty hard to meet users where they are, and tackle uncommon or emerging use cases. Examples include JSON support, support for writing functions in various languages (javascript, python, perl, R, etc.), and C extensions that can hack the engine in innumerable ways.