The first looks fine here. The second looks soft but also looks underexposed and there may be some subject or camera shake. What was the shutter speed on it? The last looks so-so. The high contrast causes color clipping and the DR of the scene is out of line with the camera's capabilities. So you have a good portion of your scene in shadow underexposed and another good chunk blown out and clipped. There are some green leaf areas that are properly exposed and they look alright to me.
The lens isn't going to be as good as your prime. That's just the nature of zooms. Also, f4 is wide open at longer focal lengths so you need to stop down for maximum sharpness. This is always the case with any lens. Some are sharper than others wide open but we're not talking about a $2000 lens here.
Try taking some more shots in some more controlled environments. Use a flash to get proper exposure throughout the image. Bump up the shutter speeds to eliminate subject movement, etc. If you still have problems it may be worth sending to Sigma but you also may want to post some full size RAW images for us to look at before going through the trouble just to rule out the possibility of the improper expectations.
Since you are by default going to get shots a little softer than your prime, one thing you may want to look at is ways to maximize sharpness in post. Just sharpening can make a pretty ugly image, but if you know how to properly sharpen it will take you a lot further. Check out the book "Real World Image Sharpening with Adobe Photoshop, Camera Raw, and Lightroom (2nd Edition)". That should help as well.
I am serious....and don't call me Shirley.
Canon 7D and a bunch of other stuff