First, all of the characters that he posted and believe to be "Mary Sues" are not. Aside from being overly perfect or some stuff like that, they never get any character development at all and most of the time are bland as f*ck not only progression wise but characterization wise especially the latter. Most of the characters that he posted have actual flaws as a character and get character development. I like it how he is ignoring this fact. If he wanted to know what a true Gary Stu/Mary Sue is, see Shadow the Hedgehog, Kirito from Sword Art Online, Edward Cullen from Twilight just to name a few.
Now can be a Mary Sue be sometimes good? Yes, if they are meant to be for comedic purposes and they are called Parody sues. This is where characters like Dante come in and heck, even Dante had character development.
