I was more asking about making a piece of the gods history unreal, let say that I cast Dismiss Into Figment on King in Yellow to make a part of his lore unreal, let say making the "He the King of Carcosa" unreal, does this make Carcosa free of the King in Yellow Reign?
Dismiss Into Figment is better at handling small, mortal targets than big complex things. Thus it's not quite a silver bullet for gods (nor can you do something like target the earth and dismiss humanity from its history.)
King In Yellow is also a tricky target for this spell, because he's already got a very loose relationship with reality.
I'd say that you can use Dismiss Into Figment on people or places to erode their relationship with him, and that you could use it on him to temporarily redact small parts of his lore, but also that he can recover from the effects of Dismiss Into Figment over time as his nature repairs the gaps in the fiction.
If multiple casters simultaneously hit him with Dismiss Into Figment while in Carcosa, that would take a serious chunk out of him. Similarly, if a large number of casters repeatedly spammed Dismiss Into Figment on Carcosa itself, they could try to break the association between Carcosa and the King and potentially free the city (even if this wouldn't guarantee the King couldn't just re-take it.)