How could it be irrational? The pump and dump can come to any coins without notice and when you see that happen, maybe you are late to join. That can happen too with Litecoin Cash but I do not think that will be the opportunity to buy because the price is already getting a pump. Maybe if you want to invest in Litecoin Cash, you need to wait for more until the price can back to the lower price. But if you're going to invest right now, you can buy it directly and hold it until the pump comes again.