From osweetman on ArcGIS Idea
There's actually a undocumented "feature" that allows labels in Collector. I wish ESRI would document it, but they actually consider it to be a bug. Here's how to add labels:
1. In the desktop application, turn on labels for the feature layer you want labeled. Format them exactly how you want them displayed in Collector.
2. Publish the feature service as you normally would (we use ArcGIS Online to host our services, so no idea if this works with Server).
3. Download a map with the feature layer to Collector. The labels only display if the map is downloaded (i.e. offline mode).
4. Voila! Labels, just like we want! And pray that ESRI doesn't "fix" this "bug"! -
Hope this helps a lot of people, and do not forget to promote the idea for "Collector - Label features" on the ArcGIS Idea dedicated page !