70-200 2.8 on aps-c (Nikon)?

Is 70-200 f2.8 too long of a lens for outdoor potraits?

I'm currently looking for a lens for my crop sensor nikon body to get a "dreamy" potrait (lots of blur)

I already own the nikon 35mm 1.8 and sigma 17-50 2.8
both these lens are not giving me the dreamy look i'm looking for.

Is the 70-200 a useful focal range for asp-c sensor? Or is there other lens that could result in this "dreamy" look i'm looking for?

I will primary shoot half body but I wouldn't mind doing head shot and full body

For outdoor portraits where you have lots of room, it's fine.
The dreamy look depends on your skills and understanding of depth of field. I would work this out with your 17-50mm lens set to 50mm @ f/2.8. Shoot inanimate objects to further your study of DOF. Learn how separation between the subject and background (in reality, the ratio of background to subject distance) affects the background fuzz.
You could look at an 85mm f/1.8 or a 100mm macro at f/2.8.
Working with an APS-C sensor won't give you as much control over DOF as a full frame sensor camera.

http://dofmaster.com/ has good information on DOF, including the on-line DOF calculator.

If you can't get what you want with those two lenses, you won't either with the longer one. Getting that dreamy look relies more on your skill. Whether or not you want to get that near (with your existing lenses) or far (with that long telephoto) is a matter of taste (or situation).

The 70-200 mm f/2.8 lens is really NOT a portrait lens. It is primarily a lens used by those who shoot editorial, sports, and photojournalism. I use that lens when shooting football from the sidelines using my Nikon D500. It is a perfect match

The best portrait lens for your APS-C sensor is probably the 50 mm f/1.8 or f/1.4 or 85 mm f/1.8 or f/1.4

The out of focus area behind the subject is more pleasing when shot with a prime lens (less internal lenses and groups). It is the fewer number of such lens elements which create that creamy, out of focus background. Pring lenses generally also have a much wider lens aperture, usually two stops (f/1.4 v f/2.8)

That out of focus part of the image behind the subject is NOT blur. For clarity, blur is caused by camera or subject movement during the exposure when the shutter speed is too long

You will be using it mostly at 70mm for "normal" piortraits. At 200mm for "effect" if you want to throw the background hugely out of focus. But possibly look also at the 85mm f1.8 if blurred background is your thing.

Why would you go to such extremes when 18-55mm f/2.8 or even an 85mm f/1.8 will do what you want? It's all a matter of having the subject far enough from the background. Of course, the 70-200 f/2.8 would be great for wildlife and bird photography but… For portraits? You'd be over-killing it.