Short answer: Yes. Longer answer: discrete categorization of life is going to be messy, but that's our fault; humans are obsessed with discrete classification

Short answer: Yes. Longer answer: discrete categorization of life is going to be messy, but that's our fault; humans are obsessed with discrete classification and attempt to apply such classifications to systems which aren't so simple. Clearly monotremes are similar to other mammals in many respects but very different in others. In trying to determine whether or not platypuses are mammals, you're really question the nature of the manmade discrete category of 'mammal', not the nature of platypuses themselves. Platypuses remain the same no matter how we choose to classify them.