You can use Canned Responses in official Jira Mobile and Jira Cloud for Mac apps to insert templates.
See Jira Cloud apps for iPhone / iPad in App Store and for Android in Google Play.
Currently, due to bugs in native Jira Mobile app – Android support for Canned Responses is suspended. We're working to resolve that issue.
Canned responses are accessible through Issue screens.
Look for "Insert template" button under "Canned Responses" label – clicking it will open a dedicated screen with:
See the interaction on Android: