Here are some synonyms for lackluster:
* Bland
* Dull
* Uninspiring
* Mediocre
* Unremarkable
* Tiresome
* Listless
Examples:
* "The movie's plot was lackluster, with predictable twists and a weak ending."
* "Her performance was lackluster, lacking the passion and energy that her previous roles had."
* "The team's performance this season has been lackluster, resulting in a disappointing record."