Being a sorcery can hinder its viability because we can't play it in the same way we would use Worldly Tutor or any other Instant-Speed effect of the cycle
For instance, we wont include The Balrog, Durins BaneThe Balrog, Durins Bane, Horn of the MarkHorn of the Mark, and Bloodcrusher of KhorneBloodcrusher of Khorne
If the copied creature is copying something else, then Deceptive Frostkite enters as whatever that creature copied, with the listed exceptions
For Yshtola, it came down to a mix of popularity of the character as well as being a spellcaster, her story and character development offered a wide range of abilities and moments to pull from to lead the deck
They're popular amongst beginners and veterans alike due to their resilient nature and tendency to return to the battlefield - even after your opponent's best efforts to keep them down