You cannot say that every innovation in religion as a Bid'ah, If something new is innovated against the rules of Shariah then it's called Bid'ah.

There is no harm in celebrating Mawlid if performed within laws of shariah. Praising Nabi (sallallahu Alayhi waSallam) through Sermons, Naats will increase Eemaan & love towards Nabi (A.S.).