Hi there,
Thanks for reaching out. Export to rasters (image) formats work with fixed resolution at this time and the team is trying to bring in more controls in upcoming releases. In the meanwhile, the document from iPad syncs automatically to the desktop, so you can use the desktop Ai to manage your export settings.
I'd also request that if you could take out a moment and post this as a feature request here: https://illustrator.uservoice.com/. This way, you will keep getting all the updates related to this feature & other users can also upvote, which would help us prioritize.
Regards,
Srishti