'Looks are not the most important thing' certainly sounds better to me. Does the Polish sentence equate to the same thing - similar, for instance, when we refer to the poor using an adjectve only, eg: 'biednych' where the unwritten 'osób' is assumed. What I mean is, does 'Wygląd nie jest najwazniejszy' actually mean: 'Looks are not the most important thing.' or is it a response to something like: 'Myślę że wgląd jest ważniejszy niż inteligencja... Wygląd nie jest najwazniejszy.'
Let's have little english dictionary time (from Merriam Webster )
Simple Definition of appearance
1. the way that someone or something looks
2. a way of looking that is not true or real
3. the action of appearing : the fact that something or someone arrives or begins to be seen
nr 1= wygląd in polish
nr 2 =~ pozory
nr 3 = pojawienie się, zjawienie się, ukazanie się
"look is not the most important" is suggest, but it is wrong.
Looks are not the most important is the closest translation, but is considered wrong.
In this case, as a noun, "Looks" is always plural. The only exception is if you are talking about a particular person at a particular time. "I look this look!" You may like how they are dressed at the time. I think this is the phrase, "Looks aren't the most important." Looks is general here, not just how you or anyone else looks at any particular time.