If my opponent's Florges used its "Moonblast" attack last turn, would my Kilowattrel's "Skill Dive" attack only do 20 damage to a Benched Pokemon?
Yes, all attack damage is reduced by 30. This damage reduction applies to all Defending Pokemon (including Benched Pokemon), and also applies to any self-damage done to your own Pokemon.
Source: Scarlet & Violet FAQ; TPCi Rules Team (2023-03-30)

Relevant cards

Note: All applicable cards may not be listed below.
SV1_79
SV1_79
SV1_93
SV1_93