Is a Maw's Grave player still considered a Shadowlands Horde player for cards that check that (like Monkey Man xp)?
The Maw's Grave and Monkey Man xp
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Is a Maw's Grave player still considered a Shadowlands Horde player for cards that check that (like Monkey Man xp)?
No.
The Maw's Grave does not have a Mon or any Faction alignment at all.