The nicknames in that channel nickname list are not using Unicode (UTF-8), they are using codepages.
mIRC has been a Unicode application since mIRC v7.0, which was released in 2010, around five years ago. It fully supports Unicode, only sends and receives UTF-8, and does not support codepages.
If you are seeing codepage-based text from other users, this means that those users have not enabled UTF-8 in their clients, which they should, or that the server you are connected to is using codepages, which they should not.
Please note that this issue has been discussed many times. You can find posts on this topic, such as
this one,
this one, and
this one, by using the forums search feature to look for "unicode" or "codepage".