My local diner, Sue’s Kitchen, is actually pretty quiet even when it's packed on a Saturday morning. I mean, the booths are spread out and they have carpet, so it doesn't get that echoey loudness. Maybe it just depends on the place.
That bit about booths and carpet makes so much sense, @the_margaret. I have a spot near me with these big, high-backed booths that really soak up the sound. Even when it's full, you get your own little quiet pocket. It's a total game changer compared to places with tile floors and metal chairs.