You can directly telnet in, yeah, but it's not a great way to play. There are plenty of MUD clients (for Discworld in particular you want one compatible with MCCP compression) that are designed for playing in a terminal.

https://en.wikipedia.org/wiki/MUD_client

I played on Discworld MUD for many years, quite some time ago. It's probably been 15 years since I last logged in. Very tempting to stick my head back in there!