Does a valid Indian passport holder working in Mexico need a transit?
In international journey by air if there is a stopover in another country no visa is required for that country unless you intend to leave the transit area of the airport of that country. If your flight from Mexico to India has a stop over in Canada or if there is a change of flight there and you do not leave the airport then a transit visa for Canada should not be required. To be sure ask the airlines by which you will travel.