A while ago, I DMed a battle royale one shot for my players. I gave my players level 20 characters and let them fight each other till there was one person standing. Near the end of this game a situation occurred between me and one of my players. I wasn't sure what the appropriate ruling was in this situation, so I chose one that made sense to me and continued on. The player accepted it, but I still feel my players argument were somewhat legitimate.

The situation

During the game, one of the other players cast a wish spell. This player asked for "A powerful magical blizzard that covered the battlefield". Since the wish spell states that the DM has great latitude in ruling what occurs in such an instance and one of my players had to leave the game soon, I decided to not hold back on my players and take stats from the meteor swarm spell for this blizzard.

I took the following modification from the spell and made the all players do checks for it after each round of combat.

Each creature must make a Dexterity saving throw. A creature takes 20d6 damage on a failed save, or half as much damage on a successful one.

The arguments

The Ranger in my game asked me if he could use uncanny dodge to halve any damage from the blizzard. He thought he could since the blizzard was created by a wish spell and that could be seen as an attack.

My ruling was, the snow storm was targeted at the battlefield and not directly at him, so there was no case of an attacker hitting him. It was a blizzard being a blizzard, there just happened to be a Ranger in it.

Was this situation handled correctly? Or is there something that I might have overlooked?

He could not have used Uncanny Dodge, but Evasion would have worked (if he had that feature).

You state that you modeled the effect of the Wish after the Meteor Swarm spell (presumably dealing cold damage, though). This is, in my opinion, definitely not overpowered, since both are 9th level spells, and you risk being unable to cast Wish ever again if you use it in this way.

Since this spell requires affected creatures to make DEX saving throws, the ranger could NOT have used Uncanny Dodge, which only works on attacks - which are (generally speaking*) only attack rolls, not "attacks" in regular English.

_{* a grappling attempt is technically also a special melee attack, but you don't hit with grapple attempts, you succeed on a check. Therefore, they are not relevant for Uncanny Dodge.}

When an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you. (PHB, p. 93)

If the ranger had chosen Evasion instead of Uncanny Dodge for his level 15 feature, that could have worked:

When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail. (PHB, p. 93)

I think you should have modified your custom "Blizzard Meteor Swarm". Meteor Swarm requires DEX saves because you are dodging falling meteors. In a blizzard, however, you can't dodge snowflakes and hail and freezing wind (duh), so a CON save to resist the freezing temperatures would have been more appropriate.