I react badly to seeing should of, could of, or must of in print. Clearly, the writing is responding to the conversational sound, which is identical to should've, could've, or must've. I assume he does not read much, or would otherwise be aware of the "have" that is making the "'ve." As a percentage matter, I may be correct in my prediction that the writer does not read much. But as a high horse grammatical response I am being unfair. I am expecting a native English speaker to be aware of the underlying construction of what he says - which seems a small expectation, ne c'est pas? Yet we pedants do not recoil with anywhere near that severity to kind of or sort of used as conversational hedges. It is informal, perhaps, but no grave solecism.
One can sense the long tracing of the usage, from a precise a spear is a kind of a weapon meaning "type" through a yurt is a sort of house, meaning a borderline designation, to our current idiom he was kind of angry, meaning "partially." But that takes a bit of pondering; it is not immediately apparent when looking at the phrase "sort of."