greetings,
I concur with what Terry wrote, a comet is simply too piddly to really make an impact on the solar surface. Specifically, comets are not very dense, and break up easily when faced with a very hot atmosphere such as the Sun's.
On the other hand, what might be very interesting would be an impact of a large asteroid on the sun. With a much denser rock like or even solid iron core, there is the possibility that a large solid body could survive passing through the outer parts of the solar atmosphere. Still, for us to observe it, it would have to be massive. Remember that when you see an image of a sunspot on the sun, if it's a medium sized sunspot or larger, it's easily as big as the entire Earth!. I'm not sure we would be able to detect an asteroid or metallic body that was much smaller as it was seen against the solar disk. But it is fun to think about.