What does the obfuscated bash script on the Uniqlo shirt do?

The exact script isn't included here, so there's no way to say what it does or what it means from the description alone. A readable transcription would be the starting point.

I'd break it into individual shell commands and expansions, then work out what each produces before running anything. Bash syntax can hide a lot in substitutions and expansions, so a string that looks like gibberish may be building another command.

Don't run an unknown script just to see what happens. If the print contains something like eval or command substitution, it could execute code that isn't obvious from a quick glance.

Once you have a transcription, inspect the pieces as text first. If you decide to test it, use an isolated environment without valuable files or network access.

Obfuscated code can be more like a puzzle or a visual design than practical software. If it turns out to be inefficient, that might be part of the joke or just a consequence of squeezing an idea into a particular form.

There are also code art projects built around self-reproducing programs and text visualizations if that side of it appeals to you.