A domino mask or a bandanna Iron Fist style mask would've been better than this. Making a darker version of the actual Guardian helmet and giving him something to wear around his neck and the sides of his face under it would've been better looking than that helmet. With the helmet's current design it's painfully apparent that the production team was trying too hard.
Diggle was a badass enough character he never NEEDED a costume. Now that he has one, it needs to reflect who he is and not what the producers think he will be. A domino mask with optional night vision and/or thermal vision would've worked fine here. Diggle doesn't need a helmet, but some nice eye gear is always useful.