This is Marceline in a dress oder not?

No, Marceline uses the dress on the episode Henchman

I don't think that is a dress, it's too basic and doesn't go down far enough. I think it is just an over sized t-shirt she decided to wear.

^ yep, its an over sized T-shirt.. she wears dresses but that is not a DRESS

That's just a simple old t-shirt..

Don't think it's a dress..it looks like a really long t-shirt to me.

HOLY GLOB THAT'S A GOOD QUESTION!!! Du see, according to Marceline's appearance wikis (and there's quite a few), it's a SHIRT! I know right? But don't get the wrong idea "Oh, Marceline's not wearing pants, blah!" Well Du know how girls wear short shorts sometimes? There ya go. Hope this helped.

No I don't think it is a dress its too short... a night kleid maybe?

nope.

Yes it is a very short dress

I think it's an over-sized shirt.