If you are EV training, then yes it's a good idea as the Macho Brace doubles the amount of EVs you get.
If you are just playing the game normally it can still help. If you don't know about EVs, basically they help increase your stats a bit. So you can increase them faster holding the MB.
Your speed is lowered, which for some Pokemon can be a problem - but Ferroseed is pretty slow anyway it doesn't matter. If you have the move Gyro Ball it should increase its power a bit.
