I have an ext hdd that's 1 tb but i don't want to have to plug it in each time I use it.
Can a thumb drive work as good or will it get too hot?
Can a thumb drive work as good or will it get too hot?

Getting a USB 3 disk is still a good idea, since they tend to have better read/write speeds than USB 2. While the full potential won't be realised in a USB 2 port, it reduces the risk of being bottlenecked by the storage medium as opposed to the port.A thumb drive works just as a HDD does. Its just people recommend an HDD because a thumb drive isn't meant for constant reads or heavy writes.
Im sure if you have a good quality USB thumb drive, it would work fine. No need to get a USB 3.0 drive, the Wiiu doesnt have a USB 3.0 port.