Difference between AにB and AはB

Question

The context is this tweet:

セットリストに適当！って書いてます。笑

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?

Answer 1

I think this person (maybe a musician) just means:

セットリストに「適当！」って書いてます。笑

I just write 「適当！」("play without a plan!") on a set list. lol

Answer 2

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.

Anyway, 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.