As a graduate, is creating a log-in in the university online campus considered an acceptance of their offer?
For any normal and reputable University, no - logging into an online system is not a "got you now, sucker!" moment. 

The only way to accept an offer is to formally respond to your offer of admission to the graduate coordinator or graduate school, usually in the form of a signed document - or otherwise unequivocally answering "yes" in a digital form, such as an e-signature.