To understand what Mouly brought to comics, compare Arcade (a magazine Spiegelman edited with Bill Griffith from 1975 to 1976) and Raw (edited by Spiegelman and Mouly from1980-1991). Arcade is slightly better designed than a typical underground, but not by much. It’s magazine size and had white paper. The main editorial task of Arcade was to round up the best cartoonists and get them to focus on coherent stories (rather than engage in their penchant for dope-inspired free associations).
Raw was an entirely different animal from Arcade: Oversize and with lots of extras thrown in (torn covers, fake bubblegum cards, inserts of the early chapters of Maus). In effect, Raw brought the issue of production values to the fore. This is also the case with the books she’s edited. Mouly brought to comics some of the aesthetics of the arts and crafts wing of small press publishing (a movement that flourished in the 1970s when Mouly was forming her aesthetics). These days, it’s normal for cartoonists to expend an enormous among of energy making sure that their book design matches their content (see any recent book by Chris Ware, Seth or Ivan Brunetti). This simply wasn’t true before Mouly came into the picture.