TJF is short for "Typical Jonas Fatty"
A fangirl of the pop band The Jonas Brothers who is stereotypically fat, ugly, and has no friends. She believes that she is destined to marry one of the Jonas Brothers, and uses all of her spare time reading fanfics of them on the internet, mainly because she has no friends to occupy her time with.
Also see "Typical Twilight Fatty"
Tammi is such a TJF; she baked a Jonas Brothers cake and ate the whole thing herself.