The asset "Rigged Low Poly Cartoon Character Base Model" was rigged using Rigify. I have just made some tests here and I realise that it is not compatible with Mixamo and Rig is not working properly. To fix this issue I have just uploaded a new FBX Mixamo compatible file.
You can easily include animations in each character from this pack by separating each one in individual files and use the auto-rig tool in the mixano site by uploading the models from this pack. it is super easy!
Hope it helps!