It depends on many factors - in many cases this is true, though not always (e.g. when Darius' mana costs were raised in 10.6, his drop in winrate was relatively flat across different MMR brackets). In particular mages tend to exhibit this most, though of course it depends a lot on which champion, which spell's mana cost, and so on.
This particular case is interesting because Illaoi is one of the most (if not the most) mana-constrained fighter, so the expectation is that mana likely isn't skewed towards newer players/lower MMR players given that unlike on less mana-constrained champions, more skilled Illaoi players still have to opt into mana itemization/rune choices and still run oom with better skill usage. It is a scaling buff which tends to favor low MMR (level scaling is more impactful later in the game, games in low MMR last longer on average) so that might mean there's a light low MMR skew here for a separate reason but typically mana/level buffs don't exhibit this v...
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