Appeal is a measure of design quality, not a popularity contest. Does the silhouette alone tell you what it is and what its personality is? Are the proportions simple and clear? Johnston and Thomas compared it to a good actor holding an audience's attention — a well-designed character is simply pleasant to look at.
Overly symmetrical or overly detailed designs tend to have less appeal, because the eye doesn't know where to settle. Asymmetry, a few big simple shapes, and one distinctive feature — a pair of eyes, a signature silhouette — usually leave a far stronger impression.
In UI this shows up in the design quality of mascots, illustrations, and error-state characters. A perfectly round circle is forgettable; a slightly asymmetric bean shape with two dots for eyes reads as instantly likable and memorable.
The plain circle on the left has no distinguishing features — nothing to remember. The asymmetric bean character on the right has eyes and a mouth, breathes gently, and blinks now and then.
When to use
When designing a mascot or empty-state illustration, simplify the shape and mix in a little asymmetry. Perfect symmetry and pure geometry read as stable but rarely memorable.