In affect, you've made yourself dumb, not actually stating fact, more so lessening the burden of your actions. You're not actually stating a factual account of your intelligence and abilities but agreeing with your own low estimate of your accomplishments.
Your intelligence will be clearly evident to any observer, lying will only lower their estimate of you, regardless of whether or not your own estimate of yourself is low due to others actions, not devious intentions.
You can say you're dumb to anything. You can say you're dumb to evade responsibility. To avoid a socially uncomfortably encounter, to lessen a social responsibility. You can say you're dumb simply to be left alone.
Or you could say you're dumb because your financial objections in life do not add up to meet your fantasy occupation, when you're forced to chose a less mentally taxing profession. It's easier to say you're dumb if, on paper, you in fact look so. When really you weren't given the opportunities or your mindset did not allow for you to maneuver further in life.
You're not dumb, your situation is dumb. Those who would judge you solely on the credentials on paper are dumb, the general world view, without further analysis is dumb, never, for an instance say "I'm dumb" unless, in fact, you are verifiably dumb. Then don't have a bad attitude.
