In English , it could be similar to "Seriously..." or "Seriously now....." mattaku mo is more depressing in a way

People use "mattaku" or "mattaku mou" when murmuring against something / someone. Just like the example sentense rjcorraya writed↑.

Mattaku (まったく) means "at all", "totally" etc. For example, a few days ago when it was really really hot where I live, I learnt a phrase=> "mattaku unzaridesu" whick means "I am sick of it", "This really making me sick" etc... However, I never heard 'mattaku mo' so I am hoping someone else will help you out ^_^