It's written as 'a pretty huge understatement' and a setup for a comeback.
First, why did Wichita say to Tallahassee 'Maybe you should leave a note' at all?
IRL, in a world without zombies, an honest and polite driver would be expected to leave a note with contact information when causing a minor accident in a parking lot when the other driver is not around. Due to the massive extent of Tallahassee's parking lot damage, 'leaving a note' is a ridiculous extension of what would be a commonplace courtesy in a normal world. This goes to illustrate just how far removed our intrepid heroes are from a normal life. In other words, 'a pretty huge understatement'.
But it's also used as a setup for Tallahassee's biting comeback.
Earlier in the film, Wichita and Little Rock pack their bags one night and leave the White House after Columbus proposes to Wichita. The only explanation was a terse note left for Tallahassee and Columbus. The note had the engagement ring Columbus proposed to Wichita with attached.
Dear C + T,
We feel like total shit about leaving.
Sorry, not good at notes.
W + LR.
Columbus: Well, yeah. Yeah, 'Not good at notes' is a pretty huge understatement.
A 'Dear John' letter is a socially inappropriate method of terminating a relationship; the modern equivalent would be breaking up by text.
Wichita and Little Rock pick up a hitchhiker (Berkeley) on their road trip and Little Rock ends up stranding Wichita when she takes off with Berkeley. Not willing to fly solo, Wichita makes her way back to the White House to rejoin Columbus and Tallahassee.
Later in the film when Wichita, Columbus and Tallahassee are leaving the Hound Dog Hotel, Tallahassee has trouble maneuvering the monster truck and ends up inflicting more damage to 'The Beast' no. 3.
<Tallahassee knocks down some driveway signage and destroys The Beast no. 3 while trying to drive the monster truck>
Tallahassee: Fuck! Fuck! Fuck!
Wichita: Maybe you should leave a note.
Tallahassee: That's your department.
Tallahassee's comeback is a direct reference to Wichita and Little Rock leaving the White House in the middle of the night and leaving only a lame note as explanation.