Yes, herbs will lower happiness. In Nacrene City (Black and White) a woman in the Pokemon Fan Club will rate your pokemon depending on their happiness. In Hearthome City ( Diamond, Pearl and Platinum) a woman will again rate your pokemon in the Pokemon Fan Club. In Goldenrod City( Heart Gold Soul Silver), another woman will rate your pokemon in the house Northeast of the Bike shop.
Hapiness Rater's statements:
"It is super friendly to you! I'm a bit jealous!" - 250-255
"It is quite friendly to you! You must be a kind person!" - 200-249
"It's friendly toward you. It looks sort of happy." - 150-199
"It's quite cute." -100-149
"You should treat it better. It's not used to you." - 50-99
"It doesn't seem to like you at all. It looks mean." -0-49
Check here for Heart Gold, Soul Silver, Diamond, Pearl and Platinum.
Check here for Black and White.