First of all, do you receive a nicklist at all when entering twitch channels? If not then you're not requesting membership for it. Try adding this to your remote script file and restart mIRC.
Code:
on 1:connect: {
if ($server == tmi.twitch.tv) {
/raw CAP REQ :twitch.tv/membership
}
}
This will populate the nicklist when entering a channel. Op users will have a @ displayed in their nick.