Your blower fan pushes air through the plenum box and is routed by the blend door to either pass the ac or heater on its way to the duct work. Outside air coming in generally isn't the issue. Rips or holes in the insulation where the heater core pipes go through the firewall likely wouldn't cause anything more than potentially losing some of your airflow out into the engine compartment. I expect it won't be much since it's very tight space where the tubes route across the top of the airbox.
Possible issues could be that the blend door is not working properly (stopping before fully open hot), heater is not getting hot enough due to restriction or or faulty regulator, or air is blowing past rather than through the heater.
You can usually check the blend door actuator fairly easily. Drop the glovebox and put the fan on lowest setting. You may be able to hear the door move as you slowly turn the temp control on the dash from full cold to full hot. You may feel some vibration at the actuator if you place your hand over it. You could also pop off the actuator and see if turns the full arc. When the actuator is off, you can swing the door with a stubby flat blade screw driver to see if it's binding or only going part of the 90 degree arc.
Swetrid is right that the heater needs to be "turned on" by the thermostat, much like your ac compressor has to be pumping coolant through to get the system cold. If you don't have a temp guage you can get a fair feel for it by comparing the top radiator hose with both heater hoses by hand. They should feel about the same temp. If the heater hoses are noticably cooler than I would definitely try replacing the regulator.
On a different approach, is the lack of heat consistent on floor, vent, and defrost? If the other routes get hot but not on top then you have a different issue where a duct was misalligned or blocked when you put the dash back together.
