The issue for me with labeling "stupid" as ableist is that it's basically saying that calling people out on behaving as though they cannot think when there is absolutely nothing to indicate that this is the case is ableist. Implying that people who are less capable intellectually in some way are less worthy is ableist. Expecting people of normal intelligence without cognitive disorders to, you know, think, is not. (Which is not to say that people with developmental/intellectual/cognitive disabilities cannot think, but I recognize that they may not have the same processing abilities, or their ability to express certain things may be hampered, etc.) "Stupidity" is careless or willful thoughtlessness and/or inanity.

("Stupid" can also mean dull, senseless, or irritating, the latter two of which are probably the primary definitions when people call someone like Cheney "stupid", rather than an implication that he is unintelligent.)