waist
Explanation
In this comic, two aliens are discussing human beauty standards. One asks why human beauty standards seem to be "the same everywhere." The other explains that it only seems diverse because humans obsess over tiny minor differences like waist-to-hip ratio, but they have essentially converged on the same basic template. The alien notes: "You can have a 12-foot tongue or wings made of diamond, but humans just want to eliminate their 'problem areas.'"
The joke takes an outside perspective on human beauty standards by viewing them through alien eyes. From an alien standpoint -- where bodies could theoretically come in any imaginable form (12-foot tongues, diamond wings, etc.) -- human beauty standards are absurdly narrow. All humans are essentially the same shape, and yet they agonize over minuscule variations in proportions. What humans perceive as a vast range of body types and beauty ideals is, from a cosmic perspective, an incredibly tiny sliver of possible body configurations.
The comic satirizes human vanity and body image obsession by zooming out to a literally inhuman scale. It points out the absurdity of spending enormous emotional energy on small variations in a body plan that is, in the grand scheme of possible organisms, remarkably uniform. This is a recurring SMBC technique: using science fiction framing (aliens, robots, God) to recontextualize mundane human behavior and reveal its underlying absurdity.