It depends on the word. For "McGonagall", a person would probably have low confidence in their being able to spell it from memory, and would check if they cared, so misspelling it is strong evidence of not caring. "Cannon", on the other hand, is the correct spelling for homophone of "canon", and so it is less indicative of not caring. "Phase", another misspelling that appears in this thread, is even more indicative of ignorance rather than apathy. Confusion between "loose" and "lose" is probably interference from words like "choose", and being reminded of the correct spelling probably will help reinforce the distinction. And going outside of the misspellings in this thread, there are plenty of misspelling, such "straight laced", "shoe-in", and "tow the line", for which a person could quite easily not be exposed to the correct spelling.