Cats vomiting yellow mucus are caused by improper diet, indigestion, gastrointestinal diseases, internal parasites, etc. It is best to send the cat to the hospital for examination immediately to confirm the cause and treat it promptly to avoid delaying the treatment opportunity. Cats' gastrointestinal tracts are relatively sensitive, especially kittens under three months old, so they need to pay more attention to their diet and try to feed them some food that is easier to digest and absorb. You can usually feed some probiotics appropriately to help regulate their gastrointestinal tract and promote digestion.