If I use Ludicolo's "Enthusiastic Dance" Ability and then play down a new Basic Pokemon after I use the Ability, would that new Pokemon do 100 extra damage this turn?
Yes, it doesn't matter when the Pokemon came into play.
Source: Evolving Skies FAQ; TPCi Rules Team (2021-08-26)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH7_34
SWSH7_34