D&D is not a game of subtlety and realism. The reason non-human species are easily identifiable is narrative clarity. Art needs to be understandable and narration needs to communicate succinctly. It's the same reason every member of GI Joe or the Avengers has a unique and instantly recognizable outfit, instead of wearing any sort of standard issue uniform that might make it hard to keep track of who's who.
Is it possible for an aasimar or tiefling to try to disguise their traits and pass as human? Sometimes yes, but when it happens it's a major story point. The same as the reformed marauder trying to hide the tattoo that signals their former affiliation, for example. It's not something that should happen accidentally or by mistake.
I'd actually go the opposite route. When an aasimar or tiefling disguises themselves and passes as human, it's the "normal world." The assumption is that this character isn't treated as anything more remarkable than an elf or a dwarf. "This is a human" is something that people assume accidentally and by mistake.
When the aasimar or tiefling reveals their true nature, that's the major story point. That's when you know they're
doing something with their traits. They're choosing to show something, something a little bit vulnerable, something they know is going to get a reaction (and not necessarily a reaction they're going to want).
I think that art depicting an aasimar or a tiefling could be a moment like that - a big story point. Or, it could be a subtle thing, a thing where people will look at the illustration and maybe guess if that's really an aasimar or a tiefling. Maybe it looks mostly human, except for something a little bit off. That's kind of the point.
Because an important part of the planetouched story is that they are, at the end of the day, not really any different from a human. They've got a little something else, but part of the narrative is how that something else
doesn't actually make them much different. It doesn't elevate them above humans, or mark them as less than human. It doesn't define them visually. They have human proportions and human diversity, except for a few often-subtle elements that marks them as different to those who pay attention or spend a long time with them.
The comparison with GI Joe and the Avengers kind of falls apart because those are specific characters. Sure, specific individuals can and probably should look distinct from each other. Aasimar isn't a specific character. It's a trait that a character can have. Asking to boldly announce "I am an aasimar!" in the art is a bit like asking to boldly announce "I am left handed!" or "I have red hair!" or "I have a hitchhiker's thumb!" in the art. Sure, in some situations, it's obvious. But most of the time, it's not the thing that defines your look.
