We know armor has a cap (wich is good), but should there also be a weapon damage (melee/bows) cap also?
Or is it ok too have OP weapons (even if you don't use Alchemy/enchant/smithing circle)?

There's nothing wrong with it, it's a single player game so it doesn't hurt anyone or anything if you choose to make OP weapons and armor for that matter

Depends on your definition of op and difficulty options and mods installed. Cap wise, there is nothing but the crazy high numbers the game isn't programmed for that just cause cdts.

Having a weapon cap doesn't make the game any harder. But it certainly does wreck your mouse & hand more. Why bother with it? You can always limit your weapon damage, if you feel that it makes the game more challenging.