Skyline Cascade's triggered ability doesn't tap the creature. It can target any creature, tapped or untapped. If that creature is already untapped at the beginning of its controller's next untap step, the effect won't do anything.
2015-08-25 · Wizards of the Coast
The triggered ability tracks the creature, but not its controller. If the creature changes controllers before its first controller's next untap step has come around, then it won't untap during its new controller's next untap step.
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SeaToSky Games · Condition LP