Is Baby Yoda actually Yoda?

The short answer is that “Yoda” is the name of a character, not a species, and the character we see on The Mandalorian is different from the Yoda we know and love from the Star Wars movies. Baby Yoda is not a younger Yoda the way Anakin Skywalker is a younger Darth Vader.

Where in the Star Wars timeline does The Mandalorian take place?

The Mandalorian takes place around 9 ABY (after the Battle of Yavin, as depicted in A New Hope). We know this because Favreau confirmed that the show takes place five years after Return of the Jedi, whose Battle of Endor takes place in 4 ABY, i.e., four years after the destruction of the first Death Star.