I think it will be easier to understand with a reordering of the sentence (and a couple of parenthetical aids): Their time passed (in conversation with) pompous nothings on his side, and civil assents on (the side) of his cousins, till they entered Meryton.
And then you go to Cagey's post below for your answer.
Thank you for the context. (Many of us are always happy to have a chance to discuss Jane Austin. )
"Pompous nothings" is a description of the things he said. They were pompous and they were about nothing. Each thing he said was a 'nothing'. As he said more than one of them, together they are 'nothings'.