I've been thinking about this for a few days and might have some suggestions that could help.
First, I agree with Graybeard's suggestion that this may be postpartum depression, so keep that possibility in mind.
But I'm pretty sure what I'm thinking will apply in either case. Start at the beginning: does she know what is expected of her, both as a member of the household and as a mother who plays with her child?
Regarding the household, have the expectations been stated explicitly, or is there an assumption that she will figure out what is expected of her in terms of contributing to the running of the home?
* If they have been clearly laid out and she is not meeting them, start by discussing why this isn't working - ASK HER, do not assume that you know.
* If there's an assumption that she will figure it out, and she hasn't, then you need to be very clear by stating it out loud, to her, in a calm discussion about the need for everyone in the home to participate in keeping everything working well for everyone who lives there. (For the record, this could also be an opportunity to negotiate the distribution of labor in the house among all members. This will lessen her very human tendency to be defensive by taking the focus off her behavior in particular and shifting it on to the household in general)
Regarding her role as a mother: how her family of origin raised children is going to have a tremendous impact on how she understand her job as a mother. My best friend grew up in a family that values hard work and productivity, and she didn't know how to play with a child when her first one was born. She struggled to learn how to interact playfully with her kids due to a lack of modelling behavior to follow. Assuming that your DIL is not suffering from PPD, she just may not know how to play with her baby. If you suspect that's the case, I would suggest that you start by simply demonstrating how to do so - not in an obvious way, but with some consistency.
Example: when my granddaughter would cry from teething, in front of her parents I would walk the floor with her and just do the comforting noises you make with a crying child, but I would also kind of talk to her parents through her, like so: "Oh sweetie, its hard to be the baby. All these grownups can't figure out what you need and all you can do is cry harder until we figure it out. We're trying baby, we're trying hard." I just talk to Zoey as if she can understand me, all the time, and I turn everything into a bit of a game, and I often notice her parents picking up those behaviors. Your DIL may not yet realize that parenting can be FUN. She may be scared of doing the wrong thing, she may be uncomfortable acting silly in front of adults, she may not understand what her baby needs beyond his physical needs. I'd try to demonstrate rather than tell in this case, but telling may have to be part of it. Model first and see if that helps. Talk to him while you play with him, or about whatever you are doing; his parents can "hear the lesson" without being given a lesson. That also helps them gain confidence as parents. Of course you should help when they ask directly for advice, but you can also do so indirectly and not make them feel incompetent. DO NOT GET CARRIED AWAY with this - it requires subtlety and they should never get the sense that you are judging or criticizing them as parents or as people.
Finally, be clear in your own mind about what you think she should be doing differently and why you think so. If you're bothered because (for instance) you don't think she's using her time productively, you need to be able to give specific examples of what you would prefer. "Do something, anything" sounds like a reasonable plea until you think about it - from her perspective, it's quite likely that she IS doing something, it's just something you don't approve of. Making this discussion productive can get very tricky, so tread carefully and know what your expectations actually are before trying to have a conversation about it.
Hopefully, you can find some bits of useful ideas in there.