The context is this tweet:
I could understand more easily if は was used instead of に. How does に change the meaning?
Does an inference have to be made here to understand what is appropriate?
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I think this person (maybe a musician) just means:
I just write 「適当！」("play without a plan!") on a set list. lol
に means that the action is being directed at the subject in question.
は means... well,
は is defined as a "topic marker", but trying to really define its usage is a whole topic of its own.
適当【てきとう】 in this context means "flippant", and so the speaker is being flippant toward the set list. In English I think the more natural way to say it would be that the speaker is being flippant about the set list.
セットリストは適当 , in this situation, would mean that the set list itself is flippant somehow, but it would be vague about why, who made it that way, and how exactly.