People seem to either love it or hate it. Well, maybe not love, but some of us accept the premise of it and have gone on to write and expand on it.
Personally, I think Illya as a designer works, in part because he played the part or faked the part a few times in the series. The idea that he didn't know how to dress has always been a puzzle to me because his suits are as nice as Napoleon's, and the all black merely reflects the time and culture (turtlenecks especially). Otherwise, he can be seen in some very nice clothing throughout the series.
So, now I get to the heart of the matter: Illya's wardrobe.
I stand by my observation that he is a well dressed individual who charms us whether he's in a well tailored suit, a burgunday mohair jack (quite the thing in the 60's), or the iconic black turtleneck. Heck, even a white shirt with that leather holster is enough to make some of us sigh. Of course this doesn't include the frequent appearances in his boxers, but that's another post entirely. We'll have to review Napoleon's wardrobe another time. Next week it's MFU in a tuxedo.
Here is my Sunday Pictorial...

Obvious, right? White shirt, holster... exposed neck... THUD!
( Illya's closet... )