Mainland Chinese tourists are being hated for 2 reasons.

They cost more than they spend, when they move in a hotel they trash the room so badly that it costs more to clean it than the price of the stay. Some hotels have found a way to fix this, banning Chinese nationals, others check the room before they move out and ask them to pay the cleaning fees if it's really too dirty.

Disregard or disrespect for their host country, trashing historical places and being rude to local people. It wasn't reported from what I know but I saw Thai people kicking Chinese arses really bad after those Chinese began shouting at and grabbing an old Thai lady because they thought her traditional grigri pendants were too expensive and she didn't want to bargain.