I feel like "underneath" might have been a better way to word it? Underwear being "inside" sounds off.
"underneath" would kind of miss the setup for the joke. I think "inside" is fine, since the English phrase is "it's not what's on the outside, it's what's inside that counts" to refer to inner beauty.
Can argue maybe more like "inward/inner facing" or "internal." But the line could be made slightly less awkward simply by rephrasing to something like: "besides, there's something more important on a date than outer appearances... something more on the inside"
The Japanese word used in the raw is: 内面的