"Stupider" is incorrect grammar.
stu·pid ( P ) Pronunciation Key (stpd, sty-)
Slow to learn or understand; obtuse.
Tending to make poor decisions or careless mistakes.
Marked by a lack of intelligence or care; foolish or careless: a stupid mistake.
Dazed, stunned, or stupefied.
Pointless; worthless: a stupid job.
A stupid or foolish person.
Latin stupidus, from stupre, to be stunned.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada