It was Hagrid who sent a Howler to Aunt Petunia in Harry Potter and the Sorcerer's Stone, after he revealed to Harry that he was a wizard and had been invited to Hogwarts.
The Howler was a very angry and loud message from Hagrid expressing his disapproval of Petunia's treatment of Harry.
Dumbledore, while aware of Harry's situation and keeping an eye on him, didn't feel the need to intervene in this way. He likely saw it as a necessary learning experience for Harry and preferred to observe from a distance. He also knew that Hagrid's Howler would serve as a powerful message to Petunia, highlighting the dangers of denying Harry his magical heritage.