This excerpt from the linked Wikipedia article for the name abstractly summarizes it nicely.

It reached the rank of the sixth most popular name for girls in the United States in 1912 and maintained that popularity through 1920, but then its popularity dropped quickly afterward.[2]

The name Mildred was very common about a hundred years ago, but never really at any other point since. If you see the name Mildred without seeing the person in question your first thoughts will be that they are extremely old. That’s really about it.