The atmosphere has been dealing with significantly more methane influx than previously assumed. Methane outgassing by the oceans alone dwarfs anything animals could do. This means that cows belching methane into the air is irrelevant. But those contraptions sure make a nice business proposition for the one selling them.
According to Wired, a new bovine burp mask fitted with a catalytic converter catches methane expelled from cows and converts it to CO2 and water. Methane is a more powerful greenhouse gas than CO2.