WordNet records no synonym and no antonym for mess of pottage. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
anything of trivial value
“Esau sold his birthright to Jacob for a mess of pottage”
Synonyms, antonyms and the broader and narrower relations for mess of pottage come from Princeton University’s WordNet. Full sources and licences.