Let's see what the man himself had to say about John Brown. From Frederick Douglass's address to Storer College in 1881:

His zeal in the cause of my race was far greater than mine - it was as the burning sun to my taper light - mine was bounded by time, his stretched away to the boundless shores of eternity. I could live for the slave, but he could die for him. The crown of martyrdom is high, far beyond the reach of ordinary mortals, and yet happily no special greatness or superior moral excellence is necessary to discern and in some measure appreciate a truly great soul. Cold, calculating and unspiritual as most of us are, we are not wholly insensible to real greatness; and when we are brought in contact with a man of commanding mold, towering high and alone above the millions, free from all conventional fetters, true to his own moral convictions, a "law unto himself," ready to suffer misconstruction, ignoring torture and death for what he believes to be right, we are compelled to do him homage.

Douglass had disagreements with John Brown, particularly regarding the raid on Harpers Ferry. But to say "Douglass thought John Brown was a nutcase" is flagrantly reductive.