As Sammaye indicated, you need to $unwind
the Egresos
array to duplicate the matched doc per array element so you can $sum
over each element:
db.Cuentas.aggregate([
{$match: {"Usuario": "MarioCares"} },
{$unwind: '$Egresos'},
{$group: {
_id: null,
"suma": {$sum: "$Egresos.Monto" }
}}
])