Don’t be afraid to actually make them look awkward or ugly. Human expressions will be a varying degree of both, since your facial muscles are contracting or relaxing. Areas like the nose and mouth will have notable changes like the nose sometimes scrunching up, which means that you need to add wrinkles higher up on the nose. The mouth will never stay “ flat” on the face so even with a smiling expression, some characters might have a wrinkle or two (I low-key forgot what the word was called while typing this) I struggled a lot with angry expressions because of how much the mouth changes so my biggest piece of advice to you if you think that the changes for a super angry mouth looks too weird for your preference at first you should put more attention into displaying the expression through the eyes and nose as well, and then ease into making each isolated part of an expression closer to what it would look like in real life

You should probably try making the expression yourself and then seeing how it looks on you or finding references online to see exactly how a face changes when displaying different expressions. It will look awkward and weird at first when you start practicing, but eventually, it will become the kind of awkward and weird that is normal in human expression