I am hazarding a guess on the spelling of this word, but am curious as to what it means. I know miercoles=Wednesday but can’t find a definition of miercolina.
I was walking my dog today and crossed paths with a grandmother walking her toddler, and the grandmother smiled, looked down at the dog and said “Beautiful, miercolina.”
Could have been the name Merculina instead, given the context.
I didn’t read it correctly all the way through, and for a moment thought it was a nice compound word for “pile of feces”. 
But it is likely what Nava said, or perhaps other somewhat similar nickname like Macorina.
Some connection with Wednesday’s god, Mercury and the related concept, mercurial?
That’s why I said “given the context”: if the OP had stepped on the old lady’s foot, I would have gone for “euphemism”, too 
(miércoles is used as an euphemism for mierda)
In all likelihood, “Mira que linda!”, or “Look how pretty!”