Whether or not you can successfully wash it depends on what it is made of. (Rayon and acetate do not wash well.) And how it's constructed. Some interfacings shrink when gotten wet. Fabric that has been stiffened usually doesn't wash well. If this is a qualty costume costing $35 or more, has firm interfacing or none at all and doesn't crease when you crumple it,it probably can be washed. But do keep in mind that it may look less fresh especially if it has been treated to be more crisp.
My granddaughter had a Disney piglet costume which we washed often. She then had a Disney Snow White costume which I don't think we washed but yet it looked good for several years. A bit of grime lent an authentic look to it. We did spot clean it from time to time As I recall.
I would spot clean the hem using very little water if the soil bothers you. Do just a small section and let it dry so that you can see the effect. I have found that machine washing and drying works best over hand washing. The wrinkles stay in when line dried.
With the exception of acetate and rayon most man made materials wash well. It's the trim and findings (interfacing and linings) that usually prevent successful washing. I've successfully washed many dry clean only clothes. The manufacturer puts the dry clean only label on clothes to be on the safe side.