However, quality is not determined by how many people enjoy a certain thing. If it was then The Phantom Menace would be a better movie than 2001 is, considering the amount of money made by the former, the number of people that viewed it and the number of people that like it all far exceeds the latter.
You are right, quality is not determined by how many people enjoy a thing, but quality can be assumed based on a broad sample of people, some of whom like and some of whom dislike a thing.
Well you can to a reasonable degree objectively assess how well things meet their goals. For example you can say that special effcts are objectively good or bad based on how convincing they are. Note, not convincing as in realistic (e.g. sound effects in Space), but convincing as in don't pull you out of the movie. The super-imposed Jabba in ANH? Objectively bad from a SFX point of view. Similarly acting has discernible intents. Comic over the top acting, believable acting, highly dramatic classical acting... What category are the Prequels going for? Well in the cases of Anakin and Padme's romantic scenes, it's reasonable to say they are attempting to be believed by the audience, not rip you out of belief with their woodenness. Do the scenes between them seem real to human nature? No. Well, Portman's half of the screen does just about, which is credit to her skills. Or more likely credit to her ability to ignore the director, but anyway...
You can to a reasonable degree assess films objectively. Just not at the macroscopic level of "good" or "bad". But you can at a level a little below that.