【Stupid】 ,  【slow】 ,  【dull】 ,  【dense】 ,  【crass】 ,  【dumb】  are comparable when they mean conspicuously lacking in intelligence or power to absorb ideas or impressions or exhibiting such a lack.

【Stupid】 can apply to a sluggish 【slow】 -witted lack of intelligence, typically congenital or habitual, or it can apply to a usually more or less transitory benumbed or dazed state of mind that is typically the result of drunkenness, shock, or illness; although the term seldom is applied to the insane or the imbecile, it often also suggests senselessness.

【Slow】  implies stupidity manifested especially in lack of quickness in comprehension or reaction; often the term is little more than a euphemistic substitute for  【stupid】 .

【Dull】 (see also  【Dull】 vs Blunt vs Obtuse and  【Dull】 vs Humdrum vs Dreary vs Monotonous vs Pedestrian vs Stodgy ) suggests a labored slowness or sluggishness of mind that may be constitutional, or the result of lack of mental exercise, or of overwork, or of a physical condition, and is unrelieved by any hint of quickness, brightness, or liveliness.

【Dense】 implies a quality of mind that makes it impervious to ideas. Additionally it may imply qualities (as obtuseness or stolidity) that reveal lack of perception, sensitiveness, or subtlety.

【Crass】 suggests a gross unfeeling quality that makes the mind incapable of delicate mental processes (as analysis, discrimination, and evaluation) or impervious to refined or spiritual ideas.

【Dumb】  (see also  【DUMB】 1 ) is a term of contempt that may be used in place of any of the preceding terms especially when obtuseness and inarticulateness are also implied.