After spending some time capturing and analyzing "lipstick" video, it seems that the lower lip does approximately 75% of the work. Is this correct? "Muppet" motion seems more symmetrical.
Different mouths will move differently.
Some people show their top teeth more when speaking, other people show their bottom teeth more when speaking. Some people naturally speak on one side of their mouth, or a specific shape will be asymmetrical most of the time etc.
This is where the "art" comes into play. Your job is to decide how your character’s lips will move by interpreting the information from the reference and applying that to the character. That means downplaying some movements and shapes and exaggerating others.
If the character has a totally different shaped mouth than your ref (like Melvin with only 2 teeth), the "art" is in finding appealing shapes that do a good job of emulating the real thing.
If you’re up for more study, try recording a "lipstick" cam of a friend saying the same lines and look for the different relative shapes (someone who has a different looking mouth that the original reference). I think you will find it interesting to see that a different mouth will look and move similarly but will have it’s own idiosyncrasies (it will have its own relative shapes). This will be more pronounced if you record the lines with emotion (acting) vs the news reader type delivery.
Hope that all makes sense and I haven't made it more confusing haha.