'Extreme' one this time, HC!! yayayay
Joker is extremely forgettable, "his facial features?.. hhmm... well, sharp.." and that's mostly all anyone could remember. Batman, being the one fighting Jokrr on maybe a monthly basis, and he keeps files on his villains, he acts, to himself, as if he knows more than the avarage person about the clown, but he doesn't really. He sometimes forgets if Jokers hair is purple of green, cuz those are the two most prominent things about him and he just got the switched, but when he saw the clown again, he remembered everything and just kept his mouth shut. Joker is forgettable, but he tries to not be, he tries way too much and circles back to being forgettable
Joker is unmemorable, forgettable, as much as he tires not to be, as much as he does everything to do the opposite. But if Joker's not directly in front of someone, they'd not be able to describe him well, "... sharp! Sharp features he's got... and, and... pale(?) skin? He wears purple... or was it green.."
And Batman prided himself with the knowladge of everything, and since he sees Joker on such a regular basis, and he has a file on everyone, including Joker, he thinks he knows what Joker is like, how he looks like, how he thinks, what makes him tick. But if asked he to, he too would have quite a hard time trying to remember what Joker looked like, maybe even mixing up his hair color, he'd also maybe forget if Joker had motifs, or personality traits.
Joker just has that aura, forgettable, mixable with anything. And that does help his chaos part, he can be anything he wants so, but it doesn't help his want to not be forgotten, it makes it easier. He blends into the background, maybe not with his explosives, to which he looks minuscule next to (which also makes his less memorable), and maybe not with his colorful attire or personality, but with his overall feel.