Turn off pixel preview in the View menu, to resolve your view in Illustrator.
The png file is going to have that "serrated line" in the .png as it is not directly vertical or horizontal—when you export to .png you're in the world of pixels and that's how diagonal lines will look!
Maybe it's the GPU preview.
Try View > Preview on CPU
So, i have to export in JPEG?[
thanks for the answer
It doesn't matter what image format you export to, it will be made of pixels, like tiny mosaic tiles. If you want smooth clean vector lines, then you'll need to export to SVG; make sure that the destination for the file supports that format first, though.
Hope that helps.