It is very much frowned upon though it does happen. The main reason for this is that the mail sender has not fully understood where they are. Always be aware of where you are and think about the relevance of your mail to the site owners. There is nothing more certain to elicit a sharp response than a wildly off target e-mail. Whilst constructive criticism is actually encouraged the sort of comment that just pans the site is guaranteed to get a sharp response.
Some people just start an online argument, usually a stream of abuse, just for the fun of it.
These are called Flame Wars or Flames. Don't Flame unless you want loads of grief. This signature could provide your name and e-mail address and your web site details. Frankly you have no clue. Don't be abusive.