Rashi #1: Do not say that you are Pash Chor (a great, free person). Rather, you are Pashach Shachor (black). Pashach is like Ilan Pashach (Shevi'is 4:6, i.e. cleaved), "va'Yfashcheni" (Eichah 3:11).

Rashi #2: Many are around you to kill. Magor it is an expression of gathering, like "Megurei El Cherev" (Yechezkel 21:17).

Radak: Pashchor claimed to be a Navi, and said that Hashem said that there will be Shalom. Yirmeyah said, you prophesized Sheker. Hashem did not call you Pashchor, rather, Magor (fear) is around. There will not be Shalom, like you say, rather, fear all around you. It is proper to call you Magor!

Malbim: Pashchor hints to expansion and freedom. Rather, you are called Magor mi'Saviv - gathering to yourself due to fear and siege.