Friday is a blessed day - a great many ahadeeth venerate this day as the best of days - if any day of the week is honoured above others, it is Friday. We have our Jummah Noon Prayer and we give each other glad tidings on this day - it is not bidah to say Jummah Mubarak as it is not a religious practice to say it, but a common courtesy extended. It's called being polite.
Your allowed to say it there is nothing wrong in it, But IF you "consider" it "religious" practice only then it's bidah other than that it's all good. Iv come across people who think it's sunnah when it's not..