@Wordsmith
It could be one of two things.
1. The straight man of a comedy duo.
2. On some sites the reader allows users to post comments directly on the manga itself, and these comments are also called tsukkomis.
@Wordsmith
It doesn't exactly translate to "snarky comment". "Tsukkomi" is one of those words that doesn't translate well in this sense. But the general definition is something like "a retort", usually among comedy duos. Watch some videos of Japanese comedy duos, you'll understand a bit better.
@wordsmith Ever read a manga like D-Frag? It's the thing the MC does every chapter. Someone tells a joke (the boke), and someone else retorts (the tsukkomi).