How can I make a bubbly foam for coffee? So I tried making a cup of coffee, the liquid still looks like tea but I'll mess around with settings to make it look like one (of course, tips would be really helpful). But I'm more concerned on how could I make foam for the top layer (white foam) look bubbly and not smooth?
i assume you are using Blender-Guru's doughnut tutorial.
While there are more than one way to do this, easiest would be for you to re-trace the steps that you took to create particles for the sprinkles. 
if not it could be found here.

https://www.youtube.com/watch?v=jNmnPXY9UQA


Create a single sphere bubble, assign a transparent but a little whitish material.
Select the Coffee mesh &amp; Add a particle system to it: type "Hair".
Select the Bubble sphere as the object instead of the sprinkle as shown in the video.
Weight-Paint mostly around the edge where coffee meets glass on the surface.
Play around with random scale particle setting to get the desired effect.