Ork snipers are a fan nickname for Vindicare Assassins on account of the Celestial Lions losing many personnel, especially their Apothecaries, to snipers that were allegedly orks. (Despite convincing evidence to the contrary, such as them just pissing off the Inquisition Space Wolves-style.) The Inquisition subsequently taught the Celestial
Loins Lions what happens to those who mess with the Imperium's "Secret Service". It is unknown whether the Inquisition is going to learn what happens to those who dick with the Space Marines, especially Space Marines who are best buddies with the Black Templars, them of zealous, righteous rage justice.
That being said, in the Black Library books, there are Orkish snipers, such as the Ciaphas Cain book "Caves of Ice". The book "Fifteen Hours" has a Grot sniper. Unless you consider actually being a good shot a requirement for being a sniper, there are greenskins who at least demonstrate a modicum of patience to wait for their target to show up and line up their shot carefully rather than immediately charge whatever is in front of them head on.
On a sidenote, back in 3rd edition, Lootas could loot (amongst other things) Sniper Rifles and use them. They only hit on a 4+ (instead of a 2+ like everyone else using them) to represent the average Ork's poor aim.