It is an Usopp feat not the way you worded it, but being unaffected by op devil fruits coming from little girls is an Usopp feat
well apparently, Oda himself disagrees with what you'are saying, which kinda showcases a new "usopp" perspective that many might not have considered before.
I really like usopp as a character, but I don't think him being immune to things like "cuteness" is what makes usopp the character we expect, the main feat in usopp, I would say, is his unexpected, undecisive personality that usually ends up in a "good" way.
sometimes, he is a coward, but other times he showed bravery and courage that surpassed expectations.
Sometimes he is loyal, and other times he shows defiance to even his captain luffy (Water-7)
Sometimes he is not dependable, other times he is very responsible and very trust-worthy.
as I said what most of the fans like about usopp is that despite his inconsistencies, he is generally a "Good character that will eventually do the best he can for his nakama" so him being petrified for thinking a cute girl is actually cute is something totally in line with his character I presume.