Trollpastas are generally stories that are either intentionally bad, deliberately poking fun at a trope (OC stories for example), or subvert some expectation (build up the tension in a story only to have a skeleton pop out). Creepypastas can have comedic elements, but they are mainly focused on horror. A general rule of thumb, if the story is mainly meant to be funny, go to troll pasta. If it's meant to be scary, generally it belongs here.

What about Crappypastas though? Based on what I understand, are they meant to be ones that are so unforgivably bad, that they don't even come across as unintentionally/ironically humorous?

It depends on the person. Sometimes someone sets out to write an intentionally bad story, other times the author just writes a really shitty story due to their lack of experience, time spent revising, or understanding of the genre (For example: a romantic fanfic around Slenderman and Eyeless Jane with no real horror elements).

So basically they are both similar categories for stories that weren't considered good enough for this website. Huh...

There are a few categories for those types: deletion log refugee, Creepypasta Trying to Be Serious but Fails, as well as a few others. They're a bit more open with their categorization there.

Usually a Crappypasta is trying to be good but ends up horrible, while a Trollpasta is intentionally bad. For example, "Jeff The Potato" and my pasta "Pokemon Bloody Evil Scary Version" are Trollpastas, while "Jeff The Killer" and "Sonic.exe" are Crappypastas.