Is there some added benefit to building a mob farm above ground?
Solution 1:
TL;DR
It is way easier to do so and is efficient enough to supply you and others with more than enough recources.
The decision comes down to a the decision "efficiency" vs. "ease of building" obviously you want your farm to work as good as possible. To achieve that you would want as much spawning inside of it as possible. Since mobs can spawn in a area of (if i remember correctly) up to 128 block away from a player and only start despawning once they are at least 24 blocks away you would want to assure, that the only space that can spawn mobs within those 128 blocks is your farm.
To achieve this above ground is quite easy: Just have your afk spot >128 block above the ground and your farm <128 Block away from your afk spot.
To achieve this underground means you can either mine out all the blocks in the 128 block radius around you {math tells me that this would mean ~2,796,202 = 128^3 * 4/3 blocks since that is the volume of a sphere} or find and light up all the caves and the surfaces in that area.
I do have to mention that to my knowledge the spawning algorithm of minecraft starts to check for spawnable spaces for mobs at the lower end of the world and from there goes upwards, meaning that in theory you would get better spawn rates at low Y levels (Check Youtubers like Ilmango for the details). But if you just want a working farm which produces more than enough for you and a hand full of friends if you stay afk there 1 hour per week then the payoff is not even remotely worth this much effort. These things only come into play if you want optimality for the sake of it.
Solution 2:
its more efficient if built high up, Here's the explanation. The way minecraft works is, it looks for all "loaded" areas around you. and look for al "Available" places that a mob could spawn on them. so the Less loaded area there is around you the better it is, that's why its recommended to build it high up so that the ground / Caves won't be loaded. now this won't matter incase you have all of your area lit and no mobs can spawn in it. Same case if you're in an ocean or so.