while what you say is true,
i don't think it is very nice to call anyone else "dumb" and "stupid"
"careless" is a far fairer word to use in this context.
No doubt careless is a less harsh word to use. However stupid does not really mean anything too harsh as well.


stu·pid (stōō'pĭd, styōō'-)
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.