Самый умный = the smartest (Superlatives) = Le plus intelligent (Superlatifs)
Самый умный что-ли?
This is a question only in form, but in fact it is a phrase intended to calm down a person who expresses an inappropriate point of view, for example, violating the hierarchy in the group. For example, if a student is arguing with a teacher. Or the younger person is impertinent to the older one.
This can be translated as: "Keep silence! Your opinion doesn't matter now."
It's not very rude, but it expresses the speaker's annoyance.