When we look at photographs we see a lot of stuff that's there, on the surface, but we also "see" things that are not physically there. When we look at the surface and observe line, color, saturation, composition, tonality and the rest, we might feel excited by their specific qualities or not, feel drawn to look longer or feel nothing. In these terms, which are those considered by the original piece, there are not really any situations that I can think of that would suggest not pushing beyond mediocrity.
I would add, however, that we don't stop looking with our eyes, we also see with our minds and emotions and we see through ideologies and within cultural moments. In the crosshairs of all those elements, we see things that aren't there on the photographs' surface...
If we understand, however, how their work is commenting on consumerism through repetition, landscape through its desolation, society through rigid geometry, suddenly these "mediocre" images are given aesthetic life and visual spark through our minds' appreciation of how they use aesthetics to create commentary. We love their appropriation of "mediocre" aesthetics in service of a specific cultural critique.
While there's no reason to strive for the mediocre, it's worth remembering that we make a judgment of aesthetic quality with more than our eyes. The ideas, knowledge and experiences we bring to looking at an image can excite us and make us passionate about work that seemed at first glance - or to an uninformed viewer - mediocre, and we very well might change our mind about its aesthetic value the more we learn and grow as appreciators of art, as we find more ways to engage with photography other than with just our eyes, giving us a more complex relationship to aesthetics than simply a momentary assessment of form.
Ultimately it's the "anyone can make a good photograph" argument. That's true, just as anyone can write a great line of poetry. My wife, who is a pianist, has taken some shots that enrage me with jealousy. Incredibly few, however, can put together photographs into a project that unites technique, form, subject, content and context in a way that's cohesive, fresh and that carves out its own place in the current photography conversation, just as none of us can turn our line of poetry into a book of Neruda-quality poems. It's just an infinitely more complex and interesting game.