I say any opportunity for her to get to actually use the dress should be taken. There is no rule saying that girls have to wear DISNEY princess dresses. She'll definitely look and feel like a princess. My girls were flower girls in a wedding a few years ago and I loved letting them wear the dresses any opportunity that came up. I'm a firm believer in using my "good" stuff at any and all opportunities (this also includes things like good china, silver, crystal, jewelry, etc). Why keep them if you're not going to use them?
As everybody else said, make sure that shoes are comfy and to have a change of clothes for after if it's an itchy or hot or in any way uncomfortable dress. If it's all comfy them let her go to the park in it. She'll get lots of attention which I'm sure will make her feel special.