Yes you can reverse heater core hoses and the heater core nor the engine will know no difference.
It doesn't matter at all which hose goes where on the heater core as all the heater core does is allow hot coolant to flow through the heater core and back out into the engine and then it repeats the process again and again.
The hot coolant that flows into the heater core is what allows the heater core to throw out heat inside your car or vehicle.
If your heater core is not heating up then it's likely clogged and you need to flush the heater core out or get a new one.
But you can put the heater core hoses on any way you would like as it doesn't matter which way they go.