stupider is not a word. i hope you were being sarcastic.
Oddly enough, according to Dictionary.com, Stupider is indeed a word:
http://dictionary.reference.com/browse/Stupiderstu·pid /ˈstupɪd, ˈstyu‑/ Pronunciation Key - Show Spelled Pronunciation[stoo-pid, styoo‑]
Pronunciation Key - Show IPA Pronunciation adjective, -er, -est, noun
–adjective
1. lacking ordinary quickness and keenness of mind; dull.
2. characterized by or proceeding from mental dullness; foolish; senseless: a stupid question.
3. tediously dull, esp. due to lack of meaning or sense; inane; pointless: a stupid party.
4. annoying or irritating; troublesome: Turn off that stupid radio.
5. in a state of stupor; stupefied: stupid from fatigue.
6. Slang. excellent; terrific. –noun
7. Informal. a stupid person.
[Origin: 1535–45; < L stupidus = stup(ére) to be numb or stunned + -idus -id4]
—Related forms
stu·pid·ly, adverb
stu·pid·ness, noun
Dictionary.com Unabridged (v 1.1)
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.
American Heritage Dictionary - Cite This Source
stu·pid (stōō'pĭd, styōō'-) Pronunciation Key
adj. stu·pid·er, stu·pid·est
1. Slow to learn or understand; obtuse.
2. Tending to make poor decisions or careless mistakes.
3. Marked by a lack of intelligence or care; foolish or careless: a stupid mistake.
4. Dazed, stunned, or stupefied.
5. Pointless; worthless: a stupid job.
n. A stupid or foolish person.
[Latin stupidus, from stupēre, to be stunned.]
stu'pid·ly adv., stu'pid·ness n.