Learn English – What does ‘graft probe’ mean? Is it well-received word

It's a cultural thing. Even though most dictionaries probably wouldn't include hissable, I don't think most native speakers would find it a particularly odd word in OP's context.

Hissing at the villains is probably more associated with pantomime audiences, but it fits perfectly well here. Even though I doubt I've ever seen the "word" hissable before, I'm sure I pick up exactly the intended meaning. It describes an exaggeratedly despicable character in a tale, someone the whole audience can unite in booing and hissing at.

OP should note that adding the -able suffix is a perfectly standard operation in English. Not all such forms will be listed in dictionaries (this one presumably not). Nor will they all be widely accepted, but I think this one would be. It's not a neoligism, just an uncommon but acceptable inflected form of an existing word.

It's also important to note that just because hissable is an acceptable adjective for bad guys in panto/cinema, that doesn't mean it can necessarily be used in any other context at all. Here are over 1500 written instances of the word. Just glancing at the first page or two, I see the word villain a lot, and all references seem to be about fictional antiheroes.

It's an unusual construction, but I think what the writer means is to draw an analogy with a machine that is seized or clogged with 'goo'. It is a strongly metaphorical statement which is supposed to communicate the idea that popular writing is unnecessarily complicated (or constrained, or obfuscated) with too many personal anecdotes, and therefore doesn't fulfil its primary purpose, which is to educate or inform.