Can avocado substitute the 'fat' in cake baking? Can avocado substitute the usual fat in cake baking? If so, what's the ratio to other ingredients?
The simple answer is not really.

Avocados have a high fat content relative to other fruit, but they're only about 20% fat, which is much less than any other cooking fats.

If you add enough avocado to provide the fat content for a cake, you'll also be adding too much water and all the other flavours of avocado, which I assume you don't want.