Why bond energy of acetone is more though it is more polar than formaldehyde?
the more polar the bond is, the less is its bond energy


I'm not sure where you are getting this from. 

Take Acid halides as an example series. As the bond get more polar ( HI (0.4 D) &lt; HBr (0.7 D) &lt; HCl (0.9 D) &lt; HF (1.9 D) ) the bond energies follow accordingly ( HI (295 kJ/mol) &lt; HBr (362 kJ/mol) &lt; HCl (428 kJ/mol) &lt; HF (565 kJ/mol) )

As the charge separation gets larger (and therfore more polar) there is a stronger will to not have that large charge separation, as nature wants to naturally come to an equilibrium - and thus the bonds are more energetic. 

Does this clear up your confusion?