A mediocre resume cover letter may get your resume trashed before it is read. I’ve trashed resumes before reading them because of the cover letter.
It is better to send a great resume without a cover letter, than to doom it with a poor cover letter.
But a great cover letter will get you an interview. They won’t even read your resume.
This article gives seven steps to write a great cover letter. Follow them all, including step 7. Everyone wants to skip step 7, but it is important. And I am not really sold on step 4.