shallowness has 1 distinct synonym spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is deepness.
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.
lack of depth of knowledge or thought or feeling
the quality of lacking physical depth
“take into account the shallowness at that end of the pool before you dive”
Synonyms, antonyms and the broader and narrower relations for shallowness come from Princeton University’s WordNet. Full sources and licences.