Please cite your criteria.There are some episodes that have objectively bad writing. Though I don't know if you could say that Star Trek as a franchise is overall good or bad. I think each iteration has to be examined on its own. "Star Trek as a whole is objectively bad, " is just to broad of a generalization.
We have objective criteria for what makes good stories and what makes bad stories. So applying those criteria to episodes, or serialized stories would let us know if they are good or bad.