One of my child’s Maths teacher told us this. If you keep saying to your child "Why are you so careless ?", he will only remember the word "careless". You should be saying, "Be more careful next time" more often than using the word "careless", then he / she will remember to be careful.
I think a few careless mistakes are fine. Then there will be room for improvement. Even as adults, we can be careless at times. You should be more alarmed if you find that your child is not understanding the question or concept or not knowing where to start solving the problem.
The number of careless mistakes tend to decrease as they grow older. If I can categorize the careless mistakes, I think they normally fall under
Transcription or transfer error. They got the working right, but somehow when they write the answer, they copy wrongly. this is quite common for lower primary kids. so, I tell my children to do the working nearer to where they will write the answer. This is to reduce the "transfer" error.
Copy the number in the question wrongly. If it happens too many times, we need to get their eyesight tested. One way is to make them highlight (either circle, underline or use the highlighter) the numbers in the question.
Calculation error. Put your answer back into the question to check. So rather than reworking the question (which some children do as a form of checking), work backwards to see if you get the number in the original question. This one takes some training, not all children can do that.
Can also teach them to check if the answer is logical first. eg. If you add 2 numbers, the answer has to be bigger than either of the 2 numbers, if not, then, the child could have done a subtraction instead.