Wait, I'm confused. Was it ever expressly stated that Myisha climbed the mountain in order to become the aspect of twilight?
The aspect themself seems to not take things that serious..... Climbing mount Targon is a tremendously serious task. Would the aspect view someone taking a solemn/deadly journey worthy of being a literal trickster god of fun and mischief? (IMO) Not unless the whole way she was finding some kind of amusement out of the journey and cracking jokes or something ( kinda like Zoe not taking her being cornered by her teachers seriously and choosing a bouncy ball to defend herself leading to her gaining the aspect's favor ).
Also, this is a fantasy world, there are people in Runeterra born with magic. If Myisha did make the climb, Maybe her constitution was just reinforced by her inborn magic and allowed her resist the environment?
And also, reincarnation is present in Runeterra. Zoe could possibly be a reincarnation of Myisha? like Ashe and Sejuani are reincarnations of Avarosa and Serylda.
Who knows lol, its all up in the air until said other-wise by a rioter.