To tell GHC to ignore library-provided instance and use your own instead, you can wrap Map in a newtype:
newtype PrettyMap key val = PrettyMap (Map key val)
instance (ToJSON a, ToJSON b) => ToJSON (PrettyMap a b) where
toJSON (PrettyMap m) = toJSON $ toList m
Another solution is to really use OverlappingInstances
:
data MyData = ...
$(deriveToJSON ... ''MyData)
instance ToJSON (Map Text MyData) where
toJSON = toJSON . toList