My Grannie did not always miter corners and our family considers her quilts as fabulous heirlooms. I have been sewing for 60 years so mitering was learned years ago or I probably would be struggling also.
As for the premie hats, just keep making them, they are needed. There are lots of different size premies. Plus, now they are putting hats on all babies not just premies. Just be sure to use really soft yarn as premie babies' skin is very tender.
As people get older, some seem to critize more than when they were younger. I noticed this in my Mother. So take that in to consideration when you get negative remarks from the "ladies".