Ethics: donation link on github repository for an academic side project
It is ethically acceptable to ask people to give you money when they are receiving nothing in return.  The fact that the request is on a Github repository, or that it is attached to software, or that it is attached to a university project, are not particularly relevant.
You should not use university resources to ask people to give you personally money.  A personal Github account is not a university resource.
Asking for money is unlikely to lead to receiving money.
Before selling anything (like software licenses) you need to check your university policies and see if the university has a legal right to your work.