Is it illegal to cancel my mother's immigration card?

Yes ~ After you get an immigrant visa, you will be a permanent resident of Canada, but the ten-year round-trip visa will not be used, but it will not be cancelled. You can enter and leave Canada with your passport or maple leaf card in the future. For details about the ten-year round-trip visa and Canadian skilled immigrants, please visit the Canadian skilled immigrants section and visa section of the Immigration Home Network ~