An action sequence from the Chinese opera 白蛇传·情/Tale of the White Snake, which showcases Chinese opera water sleeves (水袖) and spear kicking (踢枪)

things are different now that stuff has changed

Definition of idiomatic: Using, containing, or denoting expressions that are natural to a native speaker.“he spoke fluent, idiomatic English"synonyms:natural, native-speaker, grammatical, correct.

 Pun: A joke exploiting the different possible meanings of a word or the fact that there are words which sound alike but have different meanings.  

So Mordecai is comparing his feeling for this unnamed person to a grammatically correct joke. That plus his expression in the right panel suggest this is a sad admission, that he feels that his feelings are foolish and will not be returned. A perfectly structured cosmic joke.

made a beginners guide for the twitter newbies hope this helps <3

me: *sips drink* wow this is bad

friend: is it? *sips my drink* wow yeah this is bad

friend 2: let me try *sips my drink* oh God that's terrible

friend 3: hey what's going on here

me: my drink is bad

friend 3: *sips my drink* that's bad

friend 4: the drink is bad? *sips my drink*the drink is bad

friend 5: did I hear something about a bad drink?

not my chicken ass getting left alone with the wheat while the man takes his fox across the river