Well, you can. Mongo is something we call a NoSQL database; or Not-only SQL. Essentially it's a schemaless database, so you have the freedom to store whatever you want.
I wouldn't go into the gory details, because there are enough already on Hashnode. But, I'd recommend you learn both. Some places, you need to use MySQL (RDMS), and sometimes NoSQL.
What I'd say is that you do a small, tiny course in SQL, then a fully fledged course in Mongo.