I read an article that stated that ignoring the plea of Our Lady at Fatima is a serious sin? Is this true?
No. Apparitions and their messages are considered private revelations and are not binding on anyone other than the person(s) who received them.
Catholic Encyclopedia: When the Church approves private revelations, she declares only that there is nothing in them contrary faith or good morals, and that they may be read without danger or even with profit; no obligation is thereby imposed on the faithful to believe them.
For further reading: Catholic Answers Tract: Private Revelations