When discussing an answer such as this one, it is important to remember that comic books involve a combination of the literary with the visual arts (and in this, it can be viewed as a hybrid between the two). If in a short story or a novel, you wanted to convey emotion, you would necessarily rely on description (whether physical or psychological) and dialogue, perhaps even using figurative language or metaphor to further express that emotional state. In comic books, however, that same task would be expressed through the artwork directly.
Among the comic books you described, both Hark! A Vagrant and Superhuman Mutant Academy make use of the art itself to convey emotions: facial expressions, body language, such visual cues prove as the primary locus through which characters' personalities and psychological states are expressed. Furthermore, there is also the use of contrasts between one mode of expression to another, to express radical shifts in emotion. Take, for example, "Just Don't Fly During Red April," from Hark! A Vagrant, and its use of facial expressions, expressing the growing disillusionment among the recruits, from initial excitement to horror across the course of the briefing (so that you can see, expressed clearly in the artwork, the growing disillusionment and realization of what they've signed up for on the part of the recruits). You can see this technique also in Superhuman Mutant Academy, where these kinds of emotional shifts on the part of individual characters tend to be critical in shaping the comic's humor, and are often employed as a kind of punch line.
There are even more advanced techniques present as well. To take an example from Superhuman Mutant Academy, I might suggest you look towards page 19, and this section's use of framing, as it expresses the fixation one girl has with another girl's hair, and her irrational temptation to cut it off. The second girl's hair is placed in the center of multiple panels, which serves as a powerful expression of the first girl's fixation, in such a way that the hair itself seems to command the scene. This kind of technique is impossible to replicate in a purely literary format, being more in line with the kind of visual storytelling seen in film.
In short, the hybrid nature of comic books create a very different experience than what you would get from conventional literature.