When I take the N-400 application, do I have to mention this dismissed marijuana ticket?

The citation you received indeed will need to be revealed to the USCIS in the application process (and if you failed to reveal it note that the USCIS would learn of it through the background checks it undertakes regardless of the dismissal of charges and the sealing of the record), In some instances this criminal matter may render you ineligible for naturalization and even could trigger more harsh immigration consequences, notwithstanding that the charge was dismissed and notwithstanding that the record was sealed. The USCIS treats some dismissals as the equivalent of convictions if the record indicates the material elements of the offense were acknowledged. There really is no substitute for you to consult with an immigration attorney who, after learning all of the relevant information and reviewing the marijuana offense police/court records, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.