Yes the highest note is important, but it also depends on the range of the melody. "The First Noel", for example, has a range of one octave. This is quite small for most hymns or carols. It is usually sung in D and the octave range of the melody gives notes between top and bottom D. These are comfortable for just about everyone.
The problem is that you have to get the range such that it doesn't go too high for the altos and bases, and not too low for the tenors and sopranos. This limits the workable range to not higher than about an E or F (for bases and altos) and not much lower than about B or A for tenors and sopranos. Very few people can sing a range of two octaves. Most are happy with about one and a half max.
This gives a good workable range for a melody between about the Bb below middle C and the E an octave and a bit above it. (The men will sing an octave lower of course).
The First Noel manages this brilliantly.