You might be required to mortar manhole connectors inside your structures. Maybe you are concerned about materials gathering between the pipe and the connector. You might mortar the connectors because that's the way it's always been done.
We say stop.
Connector Cannot Flex
When mortaring the inside of a manhole connector you take away one of the key characteristics of the rubber - it's ability to flex with differential settlement. By creating this rigid joint you possibly could be creating more problems such as shear load on the pipe. This load creates cracks in the pipes, which in turn creates infiltration, sinkholes, damaged streets and in the end it up costing more money in the long run.
There will always be settlement between the pipe and manhole or whatever precast structure you're working with and your best bet to prevent infiltration or ex-filtration is to use a flexible manhole connector like a PSX: Direct Drive.
No Standard for Mortaring
There are no standards when mixing the mortar for manhole connectors, so you immediately have wide ranging variables in the standard for the mortar mix.
How skilled is the person applying the mortar? What's the difference in weather from one day to the next? Is the mortar mixed exactly the same for every application?
You take these variables and then try applying it to a piece of rubber. Mortar is not going to adhere to the rubber.
So now you've created a problem of the mortar cracking, pieces falling into the system and allowing materials to gather around those chunks of concrete creating backups.
It's just not worth the added expenses.
Stop using mortar for flexible connectors. The long-term damage and expenses far exceed the upfront costs of a standardized manhole connector.