Of course Usopp is smiling smugly, once again he's in a situation Sanji has dreamed about but never achieved: Usopp gets to sleep between Robin & Nami XD
Why I say "once again"? Usopp is the Lucky Bastard on the crew, simple Oda has shown Usopp gets more luck with the girls on the crew than Sanji does, esp post-timeskip.
1-It started on SA when Nami shoved Usopp's face into her cleaveage. ch.598
2-Then there was the scene in the tank during the final fight in FI, Usopp's face gets squished against the side of Nami's breast. ch.634
3- NAmi once again presses her boobs against Usopp, this time his back, in the tunnel that collapsed during their escape on Punk Hazard. ch. 694
4- a fourth one that focused more on Robin was again at Punk Hazard, when Robin had the daydream of Zoro & Usopp swimming across the lake carrying Luffy and her on their backs. I included this because Sanji would love to have Robin daydream about him (Sanji) carrying her on his back, but instead she pictured Usopp doing so.