Is anyone ever so heavenly minded that they are no earthly good?
I think the answer is no. At least in the sense that if someone genuinely has their thoughts concentrated on God and as Jesus put it, is loving God heart soul and mind (heavenly minded) then there is no reason why they would be no earthly good. In fact they should be more earthly good than anyone else.
I think this is born out throughout history where many of the great advances in health care, science and social reform have been driven by those who are very heavenly minded.
But this must be one of those sayings where a literal interpretation is not intended.
What is meant, I think, is that some people devote themselves to a selfish view of heaven (often involving a desire for study of various religious small details) where they substitute thinking for action. Where thoughts of God should motivate to good things they are instead in love with thinking and philosophizing rather than God.
So, the root of the problem would be pride and selfishness. The prime sins of the fall (that is the first sin of Adam and Eve in the Garden of Eden). Or, as God often describes it, adultery. If we don’t devote our heart and soul to God alone then we sin.
So, I would suggest any form of religion that makes the practice of that faith into one of selfishness is failing. If you don’t have your mind on heaven then you are failing. If your thoughts of heaven are exclusively about how you can be good enough, in the practice of your faith, to get past the gates then you are failing.
It is a complicated thing. We should think about our own soul and its journey and be especially concerned about our journey into the afterlife. Yet, if that fills our mind with a preoccupation with self then we have failed. As Jesus puts it seek first the kingdom (Matt 6:33).