Stupid and incompetent are two of those words that are thrown around without much thought, thus leading to confusion. Usually, someone will make a mistake or mess up when doing something, and those watching or seeing the result will call that person stupid for the failure. Politicians are often called stupid for frequently failing to fix things like the economy, poverty, and the imbalanced Federal budget. Despite how accurate it is to say that politicians regularly and maybe even naturally fail to fix such things, it is not accurate to say that politicians are stupid because of those failures. For the job of politicians is to get policies enacted, thus calling politicians stupid because they fail to balance the Federal government's budget amounts to calling an electrician stupid when he fails to fix your plumbing.
Humans have an interesting habit of saying someone is stupid or even incompetent when that someone behaves in a stupid or incompetent way. When the derogatory label is applied, it is often inaccurate at best. The distinction lies between the general case and the particular case. While I cannot currently imagine how anyone can be generally incompetent, it is entirely appropriate to use "incompetent" to refer to particular incompetence. Similarly, stupidity is especially appropriate when used with respect to someone's ability to do X or even someone's ability to comprehend X. An artists may be totally unable to grasp the concept of angular momentum, but that artist is not stupid, he is just stupid with respect to angular momentum or incompetent with respect to physics, whichever the case may be.
Why then do so many people persist in saying that politicians are stupid or incompetent when a politician trying to fix some problem that is not in anyway in the job description of politician? I currently see two answers: either (1) some of those people want the politicians, and consequently government, to fix those problems, or (2) some of those people are trying to make a point that politicians are not competent to do anything beyond politics, so politicians are stupid when they try to change things outside of politics. Unfortunately, this distinction is lost in the rhetoric of politics.
Let us examine the first answer. Those who fit this answer to my question are the hopeful sort of people, who believe that government would be better able to ease our woes, if only we had smarter, more competent, or even more moral politicians. The hope is so often dashed when, for instance, the economy wallows along, as it is often want to do in this era of semi-central planning. A non-partisan example is that of the Federal budget; few if anyone in the Mainstream Media seems to ask why politicians talk about balancing their budget, but they continue to pass imbalanced budgets.
This first answer applies to both liberals and conservatives, because both of these groups treat politicians as a means to particular ends, such as economic prosperity, equality, liberty, or even morality. Since I am betting that many liberals and conservatives who read that sentence might think I am crazy for glossing over their issues, I will have to delve into my assertion with respect to the similarity of liberals and conservatives in a future post.
Lastly, unless I can figure out what it would mean to be generally incompetent or generally stupid, I see no reason to label politicians as stupid or incompetent without simultaneously mentioning "with respect to X".