Air plasma cutters will always produce a little bit of edge angularity, especially on small holes where the diameter to thickness ratio is high. All of the big name plasma cutter manufacturers are constantly working to reduce cut edge angularity through better torch design.
To get the best angularity possible, be sure to use the lowest amperage nozzle that can still clean cut the thickness you are working with. For example, when i cut sheet metal with my 60 amp Hypertherm, i use a 30 amp nozzle for the best angularity (typically about 3 to 5 degrees).
Also, you should cut holes at about 50% of what the recommended cut speed is.
Which plasma cutter are you using? When in doubt, swap in a brand new nozzle. Good holes require the nozzle orifice to be completely defect free.