Parents do indeed complain to Hogwarts.
Parents do complain to Hogwarts about particular issues they wish to see them address. In fact, Lupin resigned when it was mentioned over breakfast that he was a werewolf partly because parents would start sending owls to Hogwarts complaining that a werewolf was allowed to be teaching there.
“I think the loss of the Order of Merlin hit him hard. So he – er – accidentally let slip that I am a werewolf this morning at breakfast.’
‘You’re not leaving just because of that!’ said Harry.
Lupin smiled wryly.
‘This time tomorrow, the owls will start arriving from parents – they will not want a werewolf teaching their children, Harry.”
- Harry Potter and the Prisoner of Azkaban, Chapter 22 (Owl Post Again)
Additionally, parents also complain to Hogwarts about matters less dramatic in nature than whether werewolves should be allowed to teach, such as the content of the Hogwarts library.
“More than one parent has demanded the removal of this particular tale from the Hogwarts library, including, by coincidence, a descendant of Brutus Malfoy and one-time member of the Hogwarts Board of Governors, Mr. Lucius Malfoy. Mr. Malfoy submitted his demand for a ban on the story in writing:
Any work of fiction or nonfiction that depicts interbreeding between wizards and Muggles should be banned from the bookshelves of Hogwarts. I do not wish my son to be influenced into sullying the purity of his bloodline by reading stories that promote wizard—Muggle marriage.”
- The Tales of Beedle the Bard
So yes, parents do complain to Hogwarts about various matters, both small and large in scale.
But parents are not always told of events.
However, parents can only complain to Hogwarts about events if they are aware of them, and parents are not always made aware of certain events at Hogwarts. For example, the official story told by the Ministry claimed Cedric’s death was an accident.
“They stared avidly from Harry to Professor Umbridge, who had raised her eyes and was staring at him without a trace of a fake smile on her face.
‘Cedric Diggory’s death was a tragic accident,’ she said coldly.”
- Harry Potter and the Order of the Phoenix, Chapter 12 (Professor Umbridge)
Myrtle’s death also was most likely explained as an accident - Riddle says from his diary that the story given out was that the death was a freak accident. While Riddle is untrustworthy, since giving obviously false information if his claim is verifiable could expose him as lying, he is likely telling the truth about the official story.
“But the Headmaster, Professor Dippet, ashamed that such a thing had happened at Hogwarts, forbade me to tell the truth. A story was given out that the girl had died in a freak accident.”
- Harry Potter and the Chamber of Secrets, Chapter 13 (The Very Secret Diary)
In addition to the Ministry and Hogwarts not always telling parents the truth, their children may not tell them what is going on either, particularly if their parents are Muggles and have little connection to the wizarding world.
“My parents are Muggles, mate,’ said Dean, shrugging. ‘They don’t know nothing about no deaths at Hogwarts, because I’m not stupid enough to tell them.”
- Harry Potter and the Order of the Phoenix, Chapter 11 (The Sorting Hat’s New Song)
Parents do not complain about certain events more because they are unaware of them, and therefore unable to complain.