Her last name includes letters with umlauts (ö), which change the pronunciation of the word based on the consonants. With this in mind, the ö would be pronounced as 'ou' and the ü would be pronounced as a 'ue'.
Her last name includes the letter "ö" which is used in European countries including Germany, the country closest in relation to Drusselstein, where the Doofenshmirtz family came from. In fact, the name "Zupermühltienatör" doesn't exactly sound like Super Multi-Inator in German.
She is called Frau Gertrude Zupermühltienatör, which is German for Ms./Mrs. Gertrude Zupermühltienatör (The German title "Frau" is used for married and unmarried women).