You would have to ask the producers as for why they decided to go with 'Na' over 'Namae', but it is probably for the sake of having a more compact title, yeah. Aesthetics would be my guess as well.
Both, the native and the English title are displayed in their intended style (君の名は。 and your name.) over here, 4 seconds in the video:
youtube (3KR8_igDs1Y)
