When baking quick breads and cakes, can I premix wet ingredients the night before?
Sure.

With quick breads you just don't want to activate your leavened prematurely. That happens when the wet and dry mix so you're fine.

Two things to watch out for:

The eggs and buttermilk are going to be more prone to spoiling after they are removed from their containers. Additionally, the batter could pick up funny flavors from the fridge. Storing in an appropriate container in the fridge and using it the next day would avoid any issues.

The wet ingredients, including the eggs, are emulsified when they are mixed. The oil dissolves into the buttermilk with the help of the egg yolks. This emulsification is important for the texture of quick breads. The emulsion is not indestructible. If you find your oil has separated in the morning you may need to beat it again to reemulsify before combining with the dry.

Note: your title says "and cakes". Cakes are assembled with different methods such as creaming or beating egg whites. These could not be done in advance without destroying the texture of the cake.