Can I 'un-mobile optimise' my blog?

I would guess you could do that by simply reversing or reverting whatever you had done to initiate that kind of optimization unless your actual theme files have been over-written. Do you recall how you got to where you are right now?

When I downloaded the theme it was already like that

Which theme has the issue?

Parallax from Themify

Your theme comes with 1 year of support, see if you can contact Themify about this https://themify.me/contact

Thanks