I realise that just about any character design from the 90s could be put here, but Deathwing (a.k.a. 'Evil Dick Grayson from the future') was a special kind of horrible:

Then you've got my favorite disco collar, Karate Kid

Any discussion of bad George Perez character designs can't be complete without this one:



Next to this, "Disco Nightwing" looks good.

Marvel's Oddball

The thing with Nightwing is the basic form language of his costume is the same as the other trapeze aerialist superhero that DC has, i.e. Deadman.... wide high collar, v-neck and such...





Deadman even has Robin booties....


Where Perez seemed to go wrong was in the decisions he made after thinking things through-the blue color of the costume was an obvious nod to the Batman, as the darker blue of Nightwing's costumewas the same basic blue as Bat's cape and cowl as depicted in the books at the time, and the accessory design was meant to evoke the idea of feathers/wings to go with the name Nightwing, so the thought process was reasonable to achieve a costume that was true to the ircus origins, Bat-connection and character name, but the result was just too busy and cluttered in execution, and the multitone of blues and gold just added to that morass of clutter and confusion. 

-M

And, to cite the obvious, Deadman is dead, so no one's going to see him investigating a crime scene and shoot him in either the bare chest patch, yellow accents, or light blue areas.  I mean, Deadman's costume was so easy a target, a guy with a hook for a hand shot him!

If you are going to take practical functionality into consideration (like peripheral vision with a disco collar or a target on your chest) for worst design, then every super-heroine costume that has high heels and or a strapless bodice that wouldn't stay up if subjected to the kind of rigors of super-hero action it would have to endure, chainmail bikinis that protect nothing, armor and protective gear that leave the midriff uncovered-talk about easy targets-would be a candidate for bad design. Wearing a heavy cape/cloak while trying to swing from building to building where it would create wind drag, tangle with rope lines, etc. a la Batman would be bad design, and the Bat symbol itself would make a great target (despite Miller's attempt to rationalize it in DKR). While some of those costumes might be more aesthetically pleasing than Nightwing's, they are still bad designs when functionality is a part of the equation. I can't speak for the OP but I think the original query was more about bad aesthetic designs than bad functional designs, but I could be wrong. Nightwing's costume has a lot of things aesthetically wrong with it without functionality concerns, but at least there was a certain rationale to the thought process that led to those decisions, unlike say Jericho's costume or Oddball's or some of the other's on here that are aesthetically bad and there is no rhyme or reason for the design choices made other than someone in the moment thought it was cool.

-M