User blog comment:AYET/Why is Buggy not dead/@comment-24769684-20150306023547

We don't really know how effective B-Haki is against the Bara Bara no Mi; we just know it works on Logias and the Gomu Gomu no Mi.

Also, Buggy caught Mihawk by surprise and was probably more of an annoyance to him than anything else, plus there was no weapons Buggy carried that would chip Mihawk's sword. So I think Mihawk randomly slashing Buggy was just his way of saying "Get out of my face."