Yeah I've had this game since release and I still hate the Mortar for being so damn clumsy.

Does anybody know what the best tactic is to use this MF thing ?

I mean you shoot the smoke to set a location but then the enemies see it, so before you can hit the same spot they get you or they go out of that area.

Any tips that you have to make this weapon a bit better ?

Use a sniper rifle instead. Or set the surrounding area on fire and let the wind pull the fire into where they are. Or you could use a rocket launcher. The rocket launcher is practically as strong as the mortar, if I remember correctly, except it's reliable.