Feedback for a student showing improvement
Just personal opinion + some experience.

First, if it's graded and the grade shows a large jump, then I'd just leave it as is.

Second, perhaps the first homework was not a true reflection of the student's ability (sickness, etc.) Just be mindful but I trust that you have gotten a clear impression of the class by now.

I feel that it's okay to remark on the improvement if the last homework was apparently bad (say, like a fail or near-fail.) Otherwise the student may not even realize it was bad all along, and this "better" comment can be a harsh blow of reality.

My desire to remark will grow stronger if I have written some suggested improvements in the last assignment or highlighted the common errors in class and the students actually followed my advices. My rule is to pair each good remark with a reason, and each bad remark with a suggestion. So, rather than "good job!" I'd suggest "good job on [whatever done right]" in order to reinforce such positive behaviors.

And lastly, just my thought and I expect not everyone would agree with me: I don't think giving customized remark or suggestion is a sign of being unfair. If someone struggles and wants to put in the effort to perform better, I wouldn't mind giving more detailed guidance in my feedback. The important thing I always remind myself is while past records are important, we should be prepared to evaluate our students with fresh eyes as well whenever possible. Don't give the impression that we had already given the final grades in our head after the first couple classes.