No. Usually the only way a heater core goes bad is when it springs a leak. Assuming you've got coolant flowing into the core, the only thing I can think that would cause it to not be really warm is if the doors aren't sealing up and it's pulling cool air in and mixing with the warm air. So it could be a vacuum leak preventing those diverter doors from closing all the way, or sometimes crap (leaves and junk) get inside there and prevent those doors from opening and closing all the way. Do you have a heater valve? Looks something like this:
https://www.google.com/imgres?imgurl...act=mrc&uact=8