I have found that painting fluffy white clouds or shadows or gradients is highly dependent on brush type, type of paint, how dry the paint on the brush is, how dry the paint on the canvas is, etc.
I am trying to figure out how to do this consistently. Is there a type of paint best for this? Maybe one that is chalkier somehow compared to most paints that are heavy bodied?
I have not focused extensively on clouds myself, but saved this YT video for later, which shows basic techniques to produce clouds using oil paint, but probably also applicable to acrylic paints as well: https://youtu.be/moq5H1eaIzA
Hope it’s helpful to you!