Aunt Miriam frequently mispronounces Didi's name, as mentioned in the Wikipedia page about the Rugrats characters. This could be due to a few reasons such as forgetfulness, a language barrier, or simply being hard of hearing. It's also possible that Aunt Miriam's mispronunciation is a character trait meant to add humor or a unique touch to her character. The mispronunciation is not tied to any specific reason in the show, and it's implied that it's just something she does. The TV Tropes page on accidental misnaming suggests that even normal people can get mixed up with names sometimes, which could be another explanation for Aunt Miriam's behavior.