Longer forms for Fritzi could be Elfreda/ Elfrida/ Elfrieda, Wilfreda, Winifred/ Winifreda, Alfreda, Fressenda, Frideswide/ Frideswida/ Fredeswida and Winfreda.
It is very rude to make fun of names in another language––are you German?––no matter how "infantile and ugly" they sound. I, for one, think it's very cute, such a nice alternative to "Friederike".
Fritzi Ritz, aunt of comic strip character Nancy. (Fritzi was originally the title character, but Nancy took over the spotlight soon after being introduced.)
This is a gorgeous nickname, although for some reason it reminds me of those deep-fried potato cakes. I'd love to be called Fritzi, but it's a far cry from my real name, Bethanie.