A joke that is "just not funny" is a really crap joke that only the person saying it finds funny. It is quite often a dad joke or a joke that is poorly timed and inappropriate for the situation it is said in. It's different than a dry joke in the sense that
even the mostist lube couldn't make it
wet.
"Just not funny" is also a way of describing a really
salty dry person, who could really do with marinating in some moisture.
It can also be used off hand to aggregate people with residing hair lines.