[He may not understand the exact context as to why Frisk is saying this, but he's played enough video games with Alphys to understand what they mean.]

Ohhh, plus 10 defense? That's pretty good for a scarf!

[He shakes his head at their pun. Goodness, they sound like Sans.]

Even better than good, I look great! Thank you, sweetheart. You've helped make this birthday of mine stunning!