"Matter" here, he uses as an expression emphasizing that reserving judgments on people equals to have infinite hope on people. Like he mentions earlier Carraway was accused of being politician for reserving his judgements all the time but he's explaining the actual intention behind it and trying to say he means something better than keeping himself out of trouble. It is not only about he will be seeing materialistic, corrupted people in east egg, but also about Gatsby who he becomes (maybe one and only) friend of, in the future.
Carraway says right after the "reserving judgement" sentence,
that he is afraid of missing something because of having early judgement on something or someone.
To explain that,
Gatsby. If you be fast in judging, you can call him abnormal, you can call him vain or obsessive. But because of Carraway had "reserved his judgements", he saw more in Gatsby and he got to know Gatsby more than anyone.
Scott Fitzgerald is throwing a slight hint on us by saying it - what kind of friendship that Carraway and Gatsby would develop in future, when Gatsby was called "The Great" by Carraway, what made Carraway to be able to see the greatness in Gatsby. It all started from, "reserving judgments is a matter of infinite hope."
And one more reflection you can get from the sentence other than Carraway's intentions and character, is Gatsby. Gatsby is the synonym for infinite hope in this book, he delays forever to judge on Daisy, and hopes hence, loves infinitely.