It is often difficult to tell whether somebody on the internet is "stupid". Principally, this concept of stupidity is subjective; additionally, try to separate stupidity from ignorance.
Taking someone's username/handle/whatever-you-want-to-call-it can perhaps sometimes lend helpful hints (e.g. being capable of correctly spelling "Valdemar Poulsen", let alone being aware of him, might automatically suggest someone of greater intellectual capacity than someone identifying themselves as "£$%^ToKiOhOtElRoXxOrS!!!1^$£"). Personally, I have seen more evidence to support inaluct's assertion that (in some cases, not all) users that add (typically suffix) numerals to their name are somewhat less intelligent than others—this isn't to say that value judgments based on an individual's username are to be encouraged (obviously?).
Strife26, you raise a valid point by stressing that the number in your username is unique to you for reasons other than every number preceeding it was taken (or that it indicates the year of your birth... "blondegurl89"... ugh), but I'm compelled to argue that some users prefer not to capitalise their screen names for similar reasons (e.g. as has been suggested, some find it more aesthetically-pleasing, or (not similarly, but still a factor for some) because case-sensitivity is not always enforced, and people can become attached to their usernames appearing in a particular format).
[edit: I can't believe I hacked all that out just to present my take on stupidity being suggested by username styles]