Can I say "Schema.org is a kind of markup ontology language"?
I like this question! We do not get theoretical or philosophical questions here. So you are making us think. STOP IT!!
In respect that it manages data through some form of mark-up such as with an ontology typically being XML, RDF, etc., you may have something. However, it is not a database (of sorts) that covers a topic. For example, a dictionary, thesaurus, fact links, etc. In that respect, No. An ontology is used to reference and understand a subject area, while schema.org mark-up is a way of communicating a sub-set of data and not the whole data set.
Similar. But different.
You are on the right track though. In it's similarity, it is trusted and used by search engines to parse data and not parse text. In this way, you really are passing data directly to search engines as opposed to content that needs to be understood.