If a lootbox is holding back content that cannot be gained through any other means than aquring a lootbox its not ok
Lootboxes are designed to entice you to buy more and more by using low drop rates and duplicates
The "its your perogative" argument is completely null here as the issue lies in the practice of hooking people on the rush of gaining items from boxes enticing them to part with more and more money for little to nothing of value in return
The item being cosmetic also does not help as cosmetic items are hugely popular, games like overwatch thrive on thier cosmetic content and companies like blizzard knoe this and lock all of it behind gambling boxes
You may not be paying to win a match but your certainly paying for a chance to win content for your game and thats not ok, anything designed squeeze money out of you with the possibility of nothing in return for you money is not ok