Marcel Duchamp once said that “art is completed by the viewer.” I agree: whether a work of art is "good" or not is subjective. That said, studying art history, art theory, mediums/techniques, and the contemporary art world can help inform the way you think, talk, and write about art.
There are many famous works of art that are universally considered "great." What makes them great? Why are they so widely celebrated? To answer these questions, we would consider historical and cultural context, technique, resources/privilege, and more.
If you are an artist, it is up to you to decide whether or not your work is successful. It is perfectly reasonable to consider the opinions of your mentors, your peers, gallerists, etc. But only you know what your motivations are, what your intentions are, and what you are thinking as you experiment with your work. Still not sure? Analyzing and writing about your own practice is an effective way to grow and develop as an artist.
Please don't hesitate to reach out if you are interested in pursuing any of these ideas along with discussions, assignments, and texts that will help you analyze and talk about art.