Yes, it is just the original game without any of the major DLCs.
However, it does feature a few minor additions, including:
* Body Swap, which lets you change the appearance and pronouns of certain characters.
* Co-op mode, which lets you play with another person on the same computer (no network play).
* An extra cameo stage depending on the version you get. For all PC, Mac, and Linux versions (including the GOG version), you get the Battletoads stage.
(Note that, while co-op mode is included, the competitive Showdown game is not.)