Antonyms of acumen:
bluntness, dulness, obtuseness, stupidity, artlessness, greenness, guilelessness, ingenuousness, innocence, naiveness, naïveté, simplemindedness, simpleness, simplicity, unsophistication, unworldliness, brainlessness, density, doltishness, dopiness, dumbness, fatuity, foolishness, half-wittedness, mindlessness, oafishness, senselessness, slowness, stupidness, vacuity, witlessness.
Examples of usage:
  1. Men of the most devout spirit, intellectual acumen, and profound scholarship, uphold these various theories.
    "Love's Final Victory", Horatio.
  2. Featherstone had shown considerable acumen in giving Jim the corner room on the first floor.
    "Colorado Jim", George Goodchild.
  3. On perspicuity and accuracy, about sixty pages were extracted from Blair; and it requires no great critical acumen to discover, that they are miserably deficient in both.
    "The Grammar of English Grammars", Goold Brown.