The best casual games like Super Smash Bros Ultimate are Rivals of Aether II, MultiVersus and Nickelodeon All-Star Brawl 2. Below are 6, each with the reason it is here — and, at the end, what filtering to casual costs you.
Filtering to casual means you want something you can put down. That is the whole definition, and it says nothing about how deep the game is.
What you get: A game that fits around a life. Short sessions, save anywhere, no penalty for a week away.
What made Super Smash Bros Ultimate worth playing in the first place is covered on the full list. This page is the casual part of that answer, and only the casual part.
The best current alternative, built by people who learned from Melee.
Free, team-focused, and unusually good at making two-on-two the default.
The most solid non-Nintendo platform fighter, and it plays properly.
Free, cross-platform, and easy to get anybody into.
For the party half only — physics comedy, zero skill required.
Party chaos where everyone builds the level that kills them.
Momentum, sometimes. A game designed to be left is also a game that is easy to forget you were playing.
Specifically, these are strong answers for Super Smash Bros Ultimate that are not casual games, so they are not on this page:
The full list is on games like Super Smash Bros Ultimate — 12 picks, no genre filter.
Rivals of Aether II. The best current alternative, built by people who learned from Melee.
No. Super Smash Bros Ultimate released on the Nintendo Switch, but a recommendation is only useful if you can play it, so the picks above span platforms and eras. What they share is the genre and the reason.
No. Several of the deepest games there are — Tetris, Balatro, Into the Breach — are playable in twenty-minute pieces, and some of the shallowest demand an entire evening.
Often the opposite. Many are built around daily timers and streaks, which means stopping does cost you — the one thing a casual game is not supposed to do.
By hand. Each pick was written for the full Super Smash Bros Ultimate list first, on what the game actually does, and this page is the part of that list that is casual — never a genre tag lookup, which is how "best racing games like Super Mario 64" used to be a page on this site.
Or read the casual guide for where to start in the genre generally.