With Misty's phobia of bugs, this was not out of the ordinary. A lot of girls her age would react the same. Though it kinda conflicted with the rest of her tomboyish personality, a lot of people just can't stand bugs. :P She did overcome it with Ash's Caterpie personally. It goes without saying her's was the most realistic.
May and Dawn both had childhood "trauma" that caused their fear. If I remember right, May was simply circled by a bunch of Tentacool who didn't attack her from what I remember. With Dawn, she was actually zapped and embarassed by her post-zapped apearance. Dawn did overcome it and put up with being around them, and was really the only one to overcome it.
As for Iris, her ice fear seemed to be simply thrown in because of Dragon type's weakness to ice/ice types. While I understand relating to her Pokemon's feelings, this was just too over the top and felt it was just witten in there "just cause". I may be missing the humor in it, but I'd of atleast liked to see her have to really deal with it in some way but that never really happened.
Just my thoughts, not an expert on every little detail of the anime. :)