Well, the low-fat diet at the same time has many advantages, for mental and physical performance, general health, and longevity. And the negative mood effects can probably be easily removed with some mild mood elevators like 5-HTP and St. John's Wort, or walks in the park, or music, or whatever.
The first month on a high-fat diet one may spend in a better mood, but eventually it may bring one to an unsightly state where his mood can't be easily improved...
It seems strange that somebody would conduct a long and costly experiment finding out results of a simple-minded strategy.
Why not research what can actually work, and give that advice to people? "If you have any problems with mood on a low fat-diet, try to turn up the lights, listen to good music, and have more sex."
This article gave people a choice between being fat/unhealthy and being depressed.