Here "to unpack" is used in the school literature class sense of "to analyze in detail and discuss". Thus, "a lot to unpack" is a flippant suggestion that a comment could serve as a fruitful subject for discussion (meaning: for unfunny jokes, speculations about the commenter's mental state, socially disapproved activities, and/or political affiliation, etc.)

An "a lot to unpack" comment adds little value by itself. It's basically a verbal reaction emoji.