What does “shooting trout in a demitasse cup” mean?

I came across the phrase, “shooting trout in a demitasse cup” in the New York Times’ columnist, Maureen Dawd’s article titled Coffee Cups in Hell. Incidentally, I as a non-native English speaker often feel her writing style studded with novel and cool phrases difficult to comprehend.

Though I guess this phrase means ‘100% certain' (Please correct me if I wrong), is this an established expression? I wonder if she deliberately used this phrase in association with Mormons’ shunning caffeine.

The phrase is used in the following context:

They (Trey Parker, Matt Stone, Robert Lopez) pushed the limits at Comedy Central when they put the Prophet Muhammad in a bear suit. But as Terry Teachout wrote in The Wall Street Journal: “Making fun of Mormons in front of a Broadway crowd is like “shooting trout in a demitasse cup.” ... If the title of this show were ‘The Quran,’ it wouldn’t have opened.

Solution 1:

It comes from “shooting fish in a barrel”, which means “extremely easy”, as it is supposedly hard to miss a target in this restricted space. So, your expression is not an established idiom, but is built from this rather common phrase by replacing the barrel with the much smaller demitasse (which is a small coffee cup). So it is built to mean “even easier than shooting fish in a barrel”, with an added coffee connotation as you noted in your question.

^{As an aside, the Mythbusters have tested and validated the easiness of shooting fish in a a barrel. As another aside, French has a related idiom to mean “miss something trivial” which is “to miss an elephant in a corridor”.}