Is it possible I could ask one of my recently retired teachers to write a good recc for me? She had me for two years and I was one of her top students, pretty gutted when she retired 
Can I do that or will it be looked down upon since she no longer teaches?
I think that’s fine. Just make sure she sends the recc to your school so you can get it processed and sent out.
Uh no. @DaedricSaiyan is incorrect.
OP, if you plan on applying only using the common app, you’ll be prompted to upload the recommender’s email address. He/She will then upload the rec letter and then it’s up to you to send it to your target colleges. Asking a retired teacher is 100% fine.