A protean term that is used to convey an evaluative judgment of a thing or a state of affairs. Something that is "cooked" may be very good, very bad, or somewhere in between. The adjective may readily carry further nuances, depending on the context.
The word otherwise does not lend itself to exhaustive definition in the abstract. Its meaning in any particular situation – or whether it has a precise meaning at all – depends heavily on the speaker's subjective purpose. Ascertainment of that meaning requires considering the use of the word in its broader conversational context.
"Oh man, that class was so cooked."