I remember Sam Levine (co-creator of Penn Zero) said that the ratings for PZ are doing well. Since PZ gets around 300,000 to 500,000 views for its new episodes, I'm guessing 600,000 for Wander is actually pretty high. It's really hard for me to understand what are good and bad ratings for sub channels, especially DXD.
I think it depends on what Disney XD's targeted demographic is aiming for (Mainly Boys 2-11/6-11 and Kids 2-11/6-11; optional demographics as bonus are Adults 18-49, Teen/Adults 12-34, Girls 2-11/6-11, etc.)
And the Live +3 could be stressed on how much viewers (kids and us fans) rewatch it to add the numbers along with Online Watch Disney XD Apps Users.
It isn't like the other channels like ABC, CBS, NBC, FOX or others that usually look at Adults 18-49 at Live + Same Day only, and maybe DVR, and most certainly Online Viewing.
I heard that Frank Angones pointed that while numbers are key to know which shows have buzz, it's mainly the word of mouth, season to season quality, network interests, branding, etc. A good rule of thumb is that if we are watching, talking about it, and loving it, then the show is doing pretty good so far.
Gravity Falls' numbers at 1-2 million for recent episodes could be a VERY BIG BONUS for the demo viewers which made EXTRA buzz for Disney to make more merchandise (video game, Journal 3, etc.). Though, with Alex's plan to make it a limited series at 3 seasons or a series finale film, the way a series ends would be up to the creators (like how Phineas and Ferb ended).
As for Wander, Disney Channel obviously gave it much wider access to be aware of the characters and cartoony adventures before moving to Disney XD. My guess is that Wander's ratings for Season 2 could be steady with enough viewers at this level because Craig mentioned that the network loved the show (and part of its top priority including Gravity Falls) and are excited with new songs, new challenges, new cartoony venues, new villain, and its story arc that can have lasting appeal despite not breaking the 1 million record used by Gravity Falls, Lab Rats, and Mighty Med.
The 7D started out at 500,000 viewers in its debut, but some articles said that it has "strong" ratings at that range which kept going like that to renew a second season to debut next year (December 2 when one episode was at 260,000). Oh and it had an app that went up to 1.8 million downloads.
I think Disney XD cares more on the buzz of the shows to get fans, kids, and families talking about it (Twitter, Tumblr that most of the crew are in to interact with fans, Facebook, Disney Stores requests of merchandise, etc.) despite its half a million viewership. And with online viewings added to help better see other venues of watching legally, that adds too.
I'm no expert on how each network owned by big corps. determine which shows have more viewership, buzz, etc. but that's just my guess or close to reasonable approach to what business model Disney XD uses to ensure much buzz on each of its shows. Mainly, are the kids, familes, and fans talking about their favorite shows often and making requests for merchandises?