>You're so annoying you pitiful old man.
>I'd like to help you, but I don't know if I can.
>I thought you were nuts,
>But you're really, really, really nuts.
>Every time I move eventually you'd find me,
>And start hanging around.
>Just another lame excuse to see me.
>Man, it's getting me down.
>You know I'm actually glad to see you.
>Maybe I'm the one who's...
>Nuts.
The scene is about her confronting her feelings giving up on helping Simon, who was lost to the crown. She's not talking to Ice King in her lyrics, she's talking to Simon. The following song's lyrics Simon wrote to Marceline before losing his mind.
>Marceline, is it just you and me in the wreckage of the world?
>That must be so confusing for a little girl.
>And I know you're going to need me here with you.
>But I'm losing myself, and I'm afraid you're gonna lose me too.
>This magic keeps me alive, but it's making me crazy,
>And I need to save you, but who's going to save me?
>Please forgive me for whatever I do,
>When I don't remember you.

Your brain imprinted "Nice" in the line "I thought you were Nuts" because you're fucking retarded and didn't pay attention to the character's emotions in the scene. Rewatch it, but specifically watch Marceline as she has to come to grips with her current Simon. The cold realization that even Simon knew how bad it was getting and all he ever wanted by using the crown was to protect her in a fucked up world.