Actually mayonaise is the better of these two ideas. Mayonaise is, fundamentally, acid + egg + oil. Baking also calls for acid, eggs and oil. The problem is mainly the dosage, as well as the form in which eggs come in mayo, but in a pinch it will do fine, for the same reason that you can’t usually heat mayonaise up
there is literally nothing wrong with mayo in cake, it’s the ingredient for probably 90% of any award winning cake, especially chocolate,
Award winning cakes are mostly about presentation etc anyway. If I buy mayonnaise from the store, I don’t know what oil it has, and how many eggs, and my baking powder already has acid in it.