Can avocado substitute the 'fat' in cake baking?
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.