There should be a little popup when you hover your mouse over a nickname that is longer than the nicklist colum so you can see the whole nickname. I know you can always make it bigger to see the whole nick, but this would be helpful.
I agree this is a good idea, and would like to see it implemented in a future release of mIRC. Though, I feel there should be an option to enable/disable it, as I think some users would not find it a positive addition to the client.