I'd have to see it to judge why she deemed it slutty. depends on what the material was made out of, how short the skirt was and how revealing the cleavage was ...yes it's summer but go for sweet patterns if you're gonna wear more revealing things...it takes some of the "slutty" out of it.
if you feel that it truly was not slutty then I'd say she either doesn't have the body or the confidence to wear something somewhat revealing and so she doesn't want you wearing it either because it's gonna mean you'll get more attention than her and that's gonna make her feel bad about herself. which is natural to feel but friends shouldn't be saying you look slutty. There's a nicer way to put it lol "you might wanna wear a tank top I can see like...allll the way down your shirt" sounds abit more like something a friend should say.
You can not dress slutty. You can only act slutty..
If you are a slut already and then wearing things that show a lot of skin etc..then you are a slut but if you are not a slut and showing skin...then you are just showing skin. That is all. She sounds jealous..
Your friend is totally jealous. It's summer, what else are you supposed to wear? A winter coat and long wool pants? lol Continue wearing what is comfortable for you, if she says it again, just tell her that it is hot out.