The Sunless Citadel art is some of the best I've seen. The image of Calcryx is just beautiful, and it does a great job of illustrating what the characters are seeing: the broken chain, the absurd mounted heads, and Calcryx herself. Same with Belak and Yusdrayl. The only thing I'm unsure about is why Meepo is pictured in a somewhat threatening pose. He looks like he's about to garrote someone with that leash.
Overall, I like that the art emphasizes NPCs, encounters, and dramatic moments in the adventures. Some of the older adventures seemed to use art as decoration, and as such, it had far less practical value. "This is somewhere in the Underdark. See, there are mushrooms." or "Here's a hodgepodge of different elements found throughout the campaign." But this is stuff that you can show to the players and say, "This is exactly what you're seeing right now."
From that standpoint, the art in Yawning Portal gets a 5/5. It's high-quality and practical.