Together, the post seems to be blending with a specific pop-culture mother archetype. Fiona is beloved as a "realistic, flawed mom" figure (even though she’s a sister acting as the family matriarch). So calling her "mommy" in that context taps into both admiration for her resilience and a flirtatious or fetishized naming trend.
That post caught my eye because it combines two very different tones: "yourfavoritemommy" is a common username format in adult or GFE (girlfriend experience) content online, often playful and intimate, while "mama fiona" likely refers to from Shameless — a tough, resourceful, maternal figure who held her family together despite chaos.