The sky being blue is objective fact only if you agree about the definition of "blue", "the sky" and where you are going to observe it from. The Earth revolving around the Sun is the most convenient way to calculate the math, but is no more "objective fact" than that the Sun revolves around the Earth. In either case, the point is that you have to have shared definitions in order to agree about facts, yet we are perfectly happy to use the term "objective fact". Hence, despite the need for some shared definitions about what constitutes "bad", it is perfectly possible for certain things to be "objectively bad".