Yes. This would be important to differentiate between the associations with the Batman character and an eye catching abnormality.

It would also be interesting to see what the impact of the Joker would be. If the Joker would also elicit greater propensity to offer the seat to the pregnant woman, once could claim that Joker, being the antithesis to Batman, similarly leads one to think of the imperative to offer the pregnant woman the seat.

Finally, another important variation would be another eye catching character specific to this experiment rather than one belonging to a fictional franchise.

This would allow one to more clearly differentiate the impact between the control (just the pregnant woman) with the introduction of eye catching costumes and with the pregnant woman wearing something eye catching herself.

Another important experimental design consideration would be to avoid repeat passengers between the multiple trials.