All these things are true: (also, I study microbes, so my examples are going to be from there)
Mutations are indeed one of the main ways we get evolution.
They are indeed defined as errors, damage, abnormalities, aberration, etc.
Many mutations are neutral, creating no effect. Some are deleterious, harming the mutant cell/organism. Very few are beneficial. Often this is context-dependent: a mutation that helps a cell grow quicker in a nutrient-rich environment might reduce its survival in stress conditions.
The kinds of mutations you are talking about (XXY, X0, etc)- where an entire chromosome that is extra or missing - are very drastic, and usually harmful in humans (Down's is a classic example). But that is not universally the case: my project right now is understanding how duplication of half a chromosome helps some pathogenic fungi evade antifungal drugs. In the species I worked with previously, we knew for certain that a chromsome deletion or duplication meant almost nothing: its cells had only one chromosome, but carried 15-25 copies of it! A couple more or less made no difference. But in animals and bacteria and many fungi, chromosome copy number is usually tightly controlled. One of the possible effects of a change in copy number in humans is cancer.
I get why you don't like the use of those terms (error/abnormality). Genetics has one of the dirtiest legacies of any scientific field, and that's with a lot of stiff competition! But even though there is a lot of nuance to it, I don't think it is a wrong term. There are a lot of checkpoints in the cell specifically designed to prevent changes in chromosome number from happening, and its effects in humans are usually not good.