When I use Meowstic's "Perplexing Eyes" attack does it change my opponent's active Pokemon's weakness before I do damage?
No. Do damage first, then apply effects.
Source: Unbroken Bonds FAQ (2019-04-19), TPCi Rules Team (2019-05-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM10_80
SM10_80