For a charcter to be round, you need to be able to identify with that character and be able to see that character as a person. Both Mrs. Hale and Mrs. Peters are real women. Mrs. Hale is the gruff, older, more matronly women to Mrs. Peters' younger more dutiful wife. Are they stereotypes? Maybe, but as you read Trifles, you can identify with Mrs. Hale's remorse and guilt at not visiting Mrs. Wright more often. You can also identify with Mrs. Peters law abiding side and also with revelations about her past. As female, you actually find yourself hoping that the women don't divulge what they've found and know. That kind of bond with the story can only happen if you have round characters present.
As for the men, they are all flat characters. Their purpose is to refelct current beliefs about women and to increase the bonding between Mrs. Hale and Mrs. Peters.
The stereotypes that are present in the play are all those negative women connotations that don't seem to go away. The way the men treat the women is a stereotype. The conversation about the quilt is ridiculous to the men because they don't realize the significance. The idea that the woman should take care of the house is another stereotype. The county attorney implies that since Mrs. Wright doesn't seem to keep a clean house, she must be guilty.
But Glaspell essentailly takes each stereotype and turns it upside down. The women find the evidence. The women figure out the motive. The attention to small details is what solves the case. The small details, or trifles, that only women would notice.