Consider some of the radical men and women who could have been , and probably were, accused of being too heavenly minded.   
George Muller - spent several hours a day in prayer as he trusted God to provide for the  117 schools he established and 10,024 orphans he cared for in his lifetime...long before it was fashionable to do either.   
 David Wilkerson, Jonathon Edwards, Dietrich Bonhoeffer, Hudson Taylor, Horatio Spafford, Amy Carmichael, William Wilberforce, Corrie Ten Boom, Joni Tada,  Richard and Sabina Wurmbrand are just a few that come to mind....the list goes on and on.  
The Bible  is full of heavenly minded ordinary individuals used by God in extreme ways to fulfill His purpose. 

In fact, I think we should be praying that God  raise up ordinary men and women in our generation who are so completely consumed by and enamored with the things of heaven that in a worldly sense they are completely useless.  They are ruined for anything but God's purpose alone.

" Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind.  Then you will be able to test and approve what God's will is - his good, pleasing and perfect will.  Paul (Romans 12:2 NIV)

"Do not become so well adjusted to your culture that you fit into it without even thinking.  Instead, fix your attention on God.  You'll be changed from the inside out.  Readily recognize what he wants from you, and quickly respond to it.  Unlike the culture around you always dragging you down to its level of immaturity, God brings the best out of you, develops well-formed maturity in you"  Paul (Romans 12:2 The message)

"God deliberately chose things the world considers foolish in order to shame those who think they are wise.  And he chose those who are powerless to shame those who are powerful."
Paul (1 Corinthians 1:27)

Jesus replied "Love the Lord your God with all your heart and with all your soul and with all your mind." This is the first and greatest commandment.  And the second is like it : 'love your neighbor as yourself'.  All the Law and the Prophets hang on these two commandments"  (Matthew 22:37-40)

It just occurred to me that  "heavenly minded" could be viewed as  synonymous with being "extremist".  I could try to defend or describe the differences between the two but I think instead I'll own the term.  
In a world that has a constantly shifting definition of extreme (and what extremes are considered culturally acceptable) while it claims to value and take pride in all things moderate, I think I'm ok with being extreme.

It's true.
I am an extreme sinner.  In my own strength I'm an extreme screw up.

I have an extremely merciful God who , in His unmoderated love, came in the flesh and went to the most extreme lengths to provide a way back into an immoderate relationship and extreme reconciliation with Him.
Because of this utterly extreme exchange..I am now living under His extravagant grace, exorbitant forgiveness, excessive power, and transcendent love.

Because of that, I can live a purpose propelled, peace filled, abundant, radical, and extreme life here and now.  I can be empowered to love in extreme and irrational ways.  
I suppose by definition that would make me an extremist...and I wouldn't want it any other way.    

So don't give it another thought if someone warns you that you are too heavenly minded...just smile and know you must be doing something right.  
 It's hard to run the race well without the finish line in mind.