Having watched this with Obama: Minority candidates are held to a double standard. If they dress nicely, they're seen as elitists (which, in context, is basically another word for "uppity"). If they dress casually, they're seen as (at best) riffraff. Trudeau, Harper, Layton, Mulcair, and Duceppe all campaigned in suits, and I don't remember much in the way of populist invective here against their fashion choices.
Singh isn't just another politician in a suit, he's a self-proclaimed fashionista. Frankly I think his minority status helped him get away with something that would be more damaging to other politicians. An appearance in GQ would be a death knell for most other aspriring NDP leaders.