I don't know what sort of channel you have, but technically mIRC has no idea who is an OP and who is not when a person joins a channel.

This code below will always return FALSE:

on *:join:#: msg $chan $iif($nick isop $chan,TRUE,FALSE)

In order to detect the correct status, an alias with timer is required:

on *:join:#:.timeropper 1 2 opper $nick $chan
alias -l opper msg $2 I'm $iif($1 isop $2,opped,not opped)