If they contain constructive criticism, should you follow their advice? The value of a rejection letter as a free critique depends on whether anyone in the editorial office has actually read your submission in full, or whether your book’s concept alone triggered an automatic rejection.
If the letter seems genuine and refers to specific aspects of your writing then you should take notice.
Worth following
The letter refers to specific aspects of your writing, and the same comments could not be applied to any other book. Take notice and consider those comments in a rewrite. Ideally discuss the feedback with other members of a writing group before committing to massive changes in your text.
Probably a template
If the same sentences could be stapled to any manuscript on the pile, the letter is not a critique. It may be an automatic rejection of the concept. Do not gut your book on the strength of a form paragraph.
When they ask to see it again
Some rejection letters actually express an interest in seeing the proposal again subject to changes being made. These are always worth following up and you should do everything they ask you to do.