Roguelikes have had 2D UIs since the start (just rendered with characters), while a MUD with a 2D interface is not a MUD but an MMORPG. There's a bit of a middle ground in rich MUD clients that can reorganize the 1D stream of words visually into a dashboard, but that's as far as you can go while still calling it a MUD.

MUDs (and closely related single-user text adventure games) are a well-defined genre; you can improve various aspects of the presentation, add rich formatting, images, background music even, but the moment you stop using text as a main medium for gameplay, you're simply not doing a MUD anymore.

I definitely saw MU* games with graphics schemes in the 90s, although most of the moment to moment activity happened in standard trulytext base "rooms". For instance there were a series of Battletech MU* games that would replicate cockpit HUDs, the hex maps, and related via ASCII art for you while piloting a mech.