Mucus does get a bad press, but only when produced in excess, and where this is largely a protective measure by the body such as the heavy mucus produced in response to Bronchitis and some other diseases.
A popular health proponent of yesteryear was a chap called "Professor Arnold Ehret" who published his "Mucusless Diet Healing System" which claimed to heal disease.
However, there is no doubting that some foods are apparently "excess-mucus-forming" such as hard-cheeses and some starch foods such as white-bread: best avoided as they can clog the system with this excess mucus laying the foundation for allergies and other such health problems.