If you can get the game + season pass for the price you were willing to play for the main game alone, then it's worth it.
It's just more of the same type content that you find in the main game — if the pricing for getting it piecemeal is really cheap, you can get only the things you want (challenge maps, throwback costumes, retro Batmobiles, side-missions, etc.). Which is ... okay. Nothing about the season pass content holds up that well on its own — none of it feels like another "episode" of Batman, outside of the Batgirl DLC (which is pretty barebones without adding any substantially new gameplay or stories).
And therein lies the real tragic part of the way DLC has been treated in this series. Having the best video game adaptation of Batman's abilities, arsenal and movement be locked to essentially one big, yet straightforward story with only tiny tidbits of other stories being thrown in is a missed opportunity. In all Arkham games, the experiences are pretty much the safest, most straightforward type of Batman stories that rarely go anywhere unexpected, both gameplay-wise and from a story perspective.
Episodic content for a Batman game where each villain or storyline gets its due diligence could be amazing. The side-missions in City, Origins and Knight feel like challenge maps strung together at best — most of them in City and Origins barely reach on that level. The gameplay is great, yet the depth of the player moveset is rarely ever explored in the main game, and it only ever gets pushed to its limits in abstract challenge modes. The gameplay could be pushed to its limits within interesting and contextualized scenarios, but it's hard to do that within the framework of a larger game.
The side-content has its moments throughout Knight and the series overall, but rather than one attempt at an "end-all, be-all," Batman story that packs everything into one overall straightforward experience, I'd much prefer three-to-four very distinct games (from thematic, story and gameplay perspectives) that are comparable to Origins' Cold, Cold Heart DLC (which was a story lifted directly from Batman: The Animated Series, naturally), each having some new spin on the gameplay experience with a succinct story arc.
It seems that's been the nature of AAA games for a while now — the mentality and/or reality of "this is the only game we're making for five years, so we can't pass up on the opportunity to have these 20 different storylines in the game, but they have to coincide with one another." I'd still love to see future Batman games that mimic the comic origins and consist of multiple individual storylines that wrap up nicely, which might have overarching stories linking it all together.