Grass being green is objective because we can measure this and prove this. Something such as artistic failure cannot be measured and proven. Everything related to quality of art is subjective.

The only thing I can say though is that we can measure and make predictions such as "Most film buffs would probably enjoy Citizen Kane" or "General movie audiences would enjoy Infinity War". So yes I will agree that we can say most people would agree with you in saying that the Room fails at being a Drama, but I wouldn't say it's objective. I think objective is such a strong word that it denotes something as being fact, implying if someone says they opposite they are saying something untrue.