I've been seeing this a fair bit lately: cover letters in which authors explain that they haven't been published, but that they're nonetheless very good at writing.
Maybe it's just coincidence, or maybe some prominent person or other recently recommended that authors should do this. Regardless: if you find yourself tempted to write such a cover letter, go do something else until the temptation passes.
As I noted a while back in my list of what not to put in a cover letter, it's not a good idea to praise yourself in your cover letter. (Listing awards you've received is, of course, fine, as long as they're relevant.)
The flip side of that (as I also mentioned in that list): don't criticize yourself or your writing in your cover letter. (I've been seeing an unusual amount of that lately too.)