Skyrim requires very little metal ability because Bethesda shows you pretty much everything and how to do it. The only thing mentally challenging are the puzzles which require a small bit of brain power to match symbols up.
You meant mental ability.
Skyrim actually needs more mental ability to play, if only because the perk descriptions are terrible and you have to figure out how some perks combo with each other.
Oblivion, with it's supposed "depth" due to having attributes was far simpler to play. Because you end up with 100 in all attributes anyway. There was hardly more planning required than getting +5 to three attributes every level up.
For those who said Skyrim is dumbed down might be correct in some areas such as reduced magic options. But I will say this, Skyrim's leveling and perk system is a step up from Oblivion's leveling and attribute system, in terms of challenging the planning ability of the player.