Do surfaces ahead of propellers decrease thrust?
does having a surface above a propeller decrease its thrust?


Anything that can prevent or reduce the acceleration of air will reduce the effective thrust of a propeller.  That might include obstructions in front of or behind the propeller.


  Why did my experiment showed different results than the blimp.


First of all, I don't see any information about the blimp.  Perhaps it too is greatly affected.  I don't know how much thrust is required in this configuration.  It may be tiny compared to the thrust available when pointing fore-aft.  If true, both propellers have reduced efficiency.

Second, I would think there's a big difference between a flat obstruction and a smoothly curving one.  Your experiment would be closer to the blimp situation if you put a large blimp shape over the propellers rather than a flat sheet.