Look for themes with clean template code without verbose constructs.
If you don’t understand a theme’s templates then move on.
Use themes with Hugo Modules only if you are willing to maintain a local Go installation and you need constant updates. Same for themes that use npm (maintaining a node.js + npm installation costs precious time).
Avoid any theme that includes jQuery. This library is mostly obsolete and insecure.
Avoid themes with SASS. libSASS is deprecated DartSASS is the new thing.
(But then again CSS has native variables that may cover your needs without the cost of a third party library.)
Keep it simple. A lot of people apparently don’t keep things simple.
A good rule is the less JS the better.
P.S. The above is my personal opinion, I am not talking either as a moderator or as a former maintainer of the Hugo Themes repo -although my time reviewing dozens of themes has certainly informed my point of view-.