I couldn't find the repository for the OpenCycleMap stylesheet (maybe it is not publicly available) but I can confirm that MTB routes are only rendered from zoom level 12 on compared to regular cycle routes which already start to show at zoom level 5. I suggest to contact [Andy Allan][1] (the author) directly.
An alternative is to use the [MTB map from waymarkedtrails][2] which also shows additional information about specific routes.
[1]: http://www.gravitystorm.co.uk/andy/http://www.gravitystorm.co.uk/andy/
[2]: http://mtb.waymarkedtrails.org