The Tammy character could have been funny in a different movie.
A Tammy-like character appeared briefly in "This is 40", and was the funniest part of the movie.
A Tammy variation was featured in "Identity Thief" and got some good laughs.
SO the pattern is, the less you see of Tammy in a movie, the more enjoyable she is. Like a spice. A movie where Tammy is the heroine is like a plate of pepper.
The Tammy character was about as realistic as Will Ferrell's character in "Elf" (he plays an elf). So if the Tammy movie was called "Bad Elf", a kind of variation on "Bad Santa" meets "Elf", it could have been enjoyable.
Maybe I missed some character development, since I shut "Tammy" off early (at the scene where Susan Sarandon discusses her rock star conquests, sounding like La-La-La-Lavinia from the Banger Sisters), but by then it was too late.