A person who talks a lot, especially about him- or herself, and who cannot keep secrets; an indiscreet or boastful person.
He is/has such a big mouth.
He went and opened his big mouth and told them the whole story.
I’m sorry. I wasn’t supposed to tell you! Me and my big mouth!
I am so sick of the sanctimony of bigmouths lecturing them about the need for civility in the wake of her murder.
From this opening we learn that Bobby is intense and serious, Ricky is a cocky bigmouth who suffers for his ego, both are pathetic and antagonistic to each other, and they know how to take quite a beating.
There is a picture of her on the site which could almost have been designed to portray a self-promoting bigmouth.