This is has been a popular topic for decades and I feel like most people get two fundamentals always twisted. Syntax and semantics. Syntax != Semantics.

Code readability = syntax that make semantics obvious.

If a block of code is written in an imperative style or functional style doesn't matter. Ask yourself instead, are the semantics clear? Are we coping or moving the values? Are we cloning the references? Are we iterating over mutable references or copies of the value?

A good language, with good code readability, makes semantics obvious. A good programmer, encourages good readability. With good readability we don't have doubts about what a piece of code is doing.