In the 7-11 promotion, Domo has a drug trip after consuming the promotional Fuji Apple Frost Slurpee. Domo can't eat apples (or presumably apple-flavored wares), which would explain the weird effect it has on him.
Domo's perpetually gaping mouth is not only to show his Big Eater tendencies, but also to illustrate his childlike wonder at the world around him.
