The criticism is not of the idea that the world has problems, and that we should look at those problems with the aim of fixing them.

The criticism is of the assumption that a world without problems theoretically could exist.

You may disagree, but you will not find a definition of such a world that everyone can agree on.

Regardless, of whether you agree (that such a definition doesn't exist) or not, if you do plan on bringing about such a utopia, and you begin to meet resistance, the question you will inevitably need to answer is: How do those who resist fit into this utopia?

The historical answer for this question, which by all appearances seems like an inevitable answer, is the reason why people criticise utopian thinking.