I can't speak to every hiring manager, but I definitely read cover letters and value non-generic ones. (And in fact my job ad instructs you to include one. So I still look at resumes that come in without one, but it doesn't speak well to your attention to detail.)
The tone doesn't matter that much, but I would avoid the extremes of very informal or very formal. It should be the first thing that I read, so if you're applying by email it should be the body of the email.
A cover letter is an opportunity for you to tell me why you're interested in this job/company specifically and to highlight things that might not be readily apparent by reading your resume. Some of the best cover letters call out specific achievements that are relevant to the job you're applying for, or preemptively address concerns that someone reading only your resume might have. Even just including enough information to show that you did some research on the job/company before applying already puts you above most cover letters. A generic cover letter makes me wonder if you're just applying to every job ad.
The tone doesn't matter that much, but I would avoid the extremes of very informal or very formal. It should be the first thing that I read, so if you're applying by email it should be the body of the email.
A cover letter is an opportunity for you to tell me why you're interested in this job/company specifically and to highlight things that might not be readily apparent by reading your resume. Some of the best cover letters call out specific achievements that are relevant to the job you're applying for, or preemptively address concerns that someone reading only your resume might have. Even just including enough information to show that you did some research on the job/company before applying already puts you above most cover letters. A generic cover letter makes me wonder if you're just applying to every job ad.