I think the base UI of Discord is fine, but having used it for almost a decade at this point, the UI has gotten worse. Besides the ads you've mentioned, they've added a lot more clutter (random icons, rarely used features, hidden menus, etc.). When I look at screenshots from 2019, I weep.

Animated server logos, colorful /gradient and tonally-varied usernames & avatars, the super emoji or whatever they're called, etc all feel like they're pushing more towards Twitch chat than anything else. Which as another commenter remarked, is essentially aligned with their original and biggest target demographic.