looking at an Alli booklet, Its reccomends 51grams LIMIT of fat a day with a diet of 1500kcals with 16g LIMIT per meal.
It depends what your doing and aiming for,
if your aiming to do a lowfat diet, eating food low in fat (not trying to reach the limit) isn't a bad idea.
But there are plent of healthy people who eat a high fat diet, but low in the 'bad' fats (saturated, trans etc)
If you just working on restricting kcals, eating less fat helps as if has double the kcals of carbs or protein per gram.
Fat, although (usually) depremental to weightloss is part of a healthy diet and isn't all bad ie:
in oily fish, in nuts and seeds, olive/flaxseed oils
you may find some meals/days you eat less than 20grams of fat a day, but on others (say they include fish or lowfat cheese etc) will be alot higher just aim that you don't go over the limit
Im rambling, but good luck