Can Arlinn Kord's -1 ability target planeswalkers?
Yes, it can. In fact the text has been errataed to say:


  −1: Arlinn, Embraced by the Moon deals 3 damage to any target. Transform Arlinn, Embraced by the Moon.


“Any target” means any creature, player, or planeswalker.

Before Dominaria, we had a rule called the planeswalker damage redirection rule: it meant when we were dealing damage to a player with a spell or ability, we could make that damage hit one of their planeswalkers instead.

Dominaria removed that rule, and any pre-Dominaria card that said “target creature or player” now says “any target” to remain functionally the same. There are a few other changes that were made too: you can read about those in Dominaria Oracle Changes under the “Planeswalker Damage Redirection” heading near the start.

If you see a card from before Dominaria that says “target creature or player”, it now has errata to say “any target” instead.