Does it apply?
Plainswalkers are not creatures, so no. A creature with deathtouch cannot destroy a plainswalker with just any amout of damage, as it could another creature.
RESOLVED
Thanks
Just some additional info that may be helpful:
There are 8 types of cards in Magic: the Gathering:
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Instant
Sorcery
Tribal
Artifact
Creature
Land
Enchantment
Plainswalker
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Artifact, Creature, Land, Enchantment, and Plainswalker are all permanent types, which means they stay on the battlefield (unlike instants and sorceries). But not all permanents are the same.
Plainswalkers are seperate from creatures, which are their own type. Nothing that only says it effects creatures can be applied to plainswalkers, unless that plainswalker somehow becomes a creature (see: {Gideon Jura}). Yes, it is possible for a permanent to be two types at once, like an artifact creature (see: {Solemn Simulacrum}). So when a plainswalker becomes a creature, it is a "creature plainswalker".
Also if something says you can effect one type, like {Path to Exile}, but the thig you want to target it multiple types, like a creature AND a plainswalker, you can still use it on it, provided that it is the type that the card specifies that it effects.
Hope I didn't just ramble a lot. Oh wait... ;D