Jester's Scepter exiles the top five cards of target player's library face down. Does this prevent them from being put onto the battlefield with Oblivion Sower's ability?
Yes. In general, "[a] card exiled face down has no characteristics" (C.R. 406.3a), including card type (C.R. 109.3), so such a card doesn't count as a land card in exile (C.R. 205.2a). Whether a player is allowed to look at that card is irrelevant here.
EDIT (Feb. 4, 2021): Edited to conform to rule update for Kaldheim.
EDIT (Feb. 4, 2021): Edited to conform to rule update for Kaldheim.