How many letters does the top answer for “a stupid incompetent person” have?

The most common answer is dimwit (6 letters).

Crossword clue: a stupid incompetent person

3 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

dimwit 6 letters wn-def

a stupid incompetent person

doofus 6 letters wn-def

a stupid incompetent person

nitwit 6 letters wn-def

a stupid incompetent person

a stupid foolish person 50% similar → nincompoop, ninny

an awkward stupid person 50% similar → gawk, lout, lummox

an incompetent or clumsy person 50% similar → duffer