You can very easily create a cryptocurrency having a fixed level of difficulty. But its uses would become very limited then. As in case of bitcoin, the difficulty level ensures that the token creation process and the confirmation process follows a predictable schedule of once every 10 minutes. If the level of complexity were to be fixed, then the block creation time would vary according to the number of miners in the network. This would have the following effects:
1. When new nodes join the network the currency creation and movement would become faster and faster leading to inflation.
2. The block confirmation time would become unpredictable. Since the currency would also slow down if miners leave the network.
So, realistically speaking, there is no upside of fixing a difficulty. The coins you'd be mining would become worthless because everybody else also gets a fixed amount for their investment. And on the other side the system would be hardly usable at all.