Mini page layout is working fine in Salesforce Standard & Custom objects.
But, I need to create Mini PageLayout for Exernal Object, which acts when hOver to the link of the record in child record It’l popup a mini page with respective record
My understanding is mini page layout only work for external objects when you hover over ‘recent items’ and not lookup fields or child records. I would love to be proven wrong here.
So you might have to either consider this as system limitation now or go for something custom (which is not a feasible approach here).