My son and I generally always coordinate. Usually him in blue, me in pink (not on purpose, because I don't think colors have a gender, it just kind of worked out that way). We have Mickey shirts this way as well as Star Wars for our upcoming star wars weekend trip. He also has a Darth Vader shirt to match my Darth Vader dress.
I try to coordinate us by either theme or color scheme because I think it looks best in pictures. And I plan equally for both of us - some times my outfit takes preference (and he just gets whatever matches) and sometimes his does. It's never bothered me that I'm wearing a Mickey shirt while meeting Pooh or anything (though I also do try to have a general idea of what day we'll be meeting what characters and that does goes into the outfit planning).
Other characters would notice my daughter's shirt (usually with Frozen characters on it) and comment and/or point. I don't think it's necessary to change into a different shirt for each character meet and greet. If anything, whatever character she decides to wear for the day will spark fun interactions.