There are many different ways one could approach the question which is the title of this thread. One such would be to ask something like "But just what do you mean by the words 'good astronomy'?" or "How do you decide what 'good astronomy' is?"

One approach is to say "If Kant's contemporaries said, about Kant's writings, that it was 'good astronomy', then it was."

And such an approach is obviously valid (and so the question in the OP relatively easy to answer).

Another approach is to examine his work in the light of what the community of (professional) astronomers today considers to be 'good astronomy'.

Yet another is to approach the question from the perspective of the history of ideas, and ask something like "To what extent did Kant's writings influence the content and direction of astronomy, of his time and in the following decades and centuries?"