Learn Swedish: Matkoman är ju hemsk. - The food coma is surely terrible.

Ju is a small conversational word (a modal particle) used when you assume the listener already agrees with you or knows what you mean. While "surely" or "of course" are common translations, in this context it really just adds a tone of shared experience, as if you are saying, "Food comas are just the worst, aren't they?"

Is matkoma a common word in Swedish?

Yes, it is a very popular, everyday compound word made from mat (food) and koma (coma). It works exactly like "food coma" in English. Because koma is an en-word, the definite form becomes matkoman (the food coma).

Why is the adjective written as hemsk rather than hemskt?

The adjective must agree with the noun it describes. Because matkoman is an en-word, the adjective takes the basic form, hemsk. You would only add the -t to make it hemskt if you were describing an ett-word (like ett monster är hemskt) or using it as an adverb (like "terribly bad").

Since this lesson is about resultative reflexives, how could I describe eating until I get a food coma using that structure?