What is intelligence and what does having a high IQ mean? Intelligence is defined as the ability to adapt to one's circumstances, or to one's environment. This would include observational intelligence, memory, adaptability, speed of learning, problem-solving, use of language/linguistic ability, and so on. Clearly we don't all perform the same on tests designed to measure how each of us does on a scale of possible scores on the test, one that is supposedly normed for the population of the persons taking the test and its inevitable variants.
(A properly designed test has gone through rigorous 'item analysis' to ensure each question/problem measures what it purports to measure and that it discriminates between various levels of what it measures. Additionally, it has at least two 'equivalent forms' of the test, usually by randomly assigning items that have been validated to either of the forms).
It was determined decades ago that IQ tests, per se, have inherent biases in them if they are not normed for the population from which the subject comes. Cultural and educational experiences, for example, or rather their paucity, can have an adverse effect on the subject's scores, and if the scores are used for 'selection' processes, say intake to universities or to government positions, you can appreciate that neither the institution nor the subject's interests will have been well served. Obviously, there are ethical problems with deselecting people whose achievement on the test falls short of a selection cut-off criterion, and you could see that even administering it would be unethical for the possible psychological outcomes one would incur when being informed of the unsatisfactory results.
Does an average score (by definition, a score falling inside of the boundaries of one standard deviation on either side of the mean score for the test in question, with the standard scores normed properly) mean non-extraordinary performance in academia or in the workplace? Not at all. A score derived on any one day is merely a guesstimate. Its counterpart generated a week later might be somewhat different, poorer or better. If better, there's the problem of the 'test-retest effect' where people learn to do the test with each exposure to it...or its variants. There is also a phenomenon known as the 'Flynn' effect, which I won't go on to discuss; you can google it. But it means such tests must be revalidated every decade or so.
Are mental illness and intelligence correlated? Yes, often. Are they causally correlated? Not by a long shot.
The western militaries have done a great deal of research on these selection tests and have successfully defended numerous legal challenges to them. In the case of the Canadian Armed Forces, the GC and later tests were tests of learning ability. That is, how quickly can the person learn so as to succeed (and not fail) to master the material he/she must learn and not waste a valuable seat on the few courses we can afford to run each year on the topic? It turns out that for many occupations a score in the 'below average' range, or two standard deviations below the mean, is sufficient to grant a place to recruits. Of course, we don't rely solely on the GC or its modern variants. Educational background, work experience, references, and the person's appreciation of what he/she seeks in the way of work are all important. The latter speaks to motivation, another exceedingly important factor in predicting success. Has she bothered to meet with and interview people doing that work? Does she know how long the course is, and what topics she'll have to learn? Does she know where she could be employed? For example, if she wants to be trained and employed as a fire fighter, how does she feel about being deployed at sea for between six months to a year? Canadian frigates and destroyers have marine helicopters, and they need to be able to put out aircraft fires...and ship's fires.
I'll stop, but I hope you understand that there's much more to the simple IQ score, and its means of measurement and its utility in selection, than might meet the eye.