Who mocks whom?
Radak: All [nations] who see Yisrael mock them.
Malbim: They mock me 1 as if I am a great nation that fell from its high level.
What is the meaning of "Yaftiru [Safah]"?
Rashi: They open [their lips]. This is like "Peter Rechem", "Peter Chamor" (Shemos 13:12-13), "Poter Mayim" (Mishlei 17:4).
Radak: They speak words of mockery and disgrace. They send the words via the lips. This is like "Poter Mayim" (Mishlei 17:4). 'Sending' applies to water - "ha'Meshale'ach Mayanim" (below, 104:10), and to words - "Picha Shalachta v'Ra'ah" (50:19). The Targum of "veha'Anashim Shulchu" (Bereishis 44:3) is 'v'Guvraya Ispataru.'
Malbim: They do not mock me like something unimportant, rather, like a great, esteemed matter.
Why does it say "Yani'u Rosh"?
Radak: They shake their heads at them.