Mediocre performance is mostly attributed to average capability. It might be acceptable to be mediocre at an unimportant job. You can find ways to get out of such tasks. But to be mediocre at simple jobs and not being excellent at higher jobs has no excuse at all.
Willingness to perform, coupled with the application of self to the job gets the best out of even mediocre people. People with greater willingness to perform, learn, stretch and work-hard to deliver better results beyond their core capability. People who are not willing, are unable to apply themselves to even the simplest tasks. Mediocre people who apply themselves go farther than the superior people who don’t.
To get the best out of mediocre members of the team, even before finding interventions to improve their performance, check how willing they are. If you can find a way to increase their willingness and application skills, they will be able to perform better than their latent best.