Technically they aren't supposed to sleep in those things but I had a Rock 'n' Play (same idea as a mamaroo) and let my LO sleep in it until she was 4 months old.
Does your LO always sleep with her head at that angle? My daughter had a tight neck (torticollis) so she developed a flat spot from sleeping at the same angle all the time. Just some thoughts.
My LO sleeps in her glider (which is the same shape as the mommaroo) during the day. I was scared at first but she's totally fine. I make sure to turn it off once she's asleep so she doesn't get used to the movement while sleeping. The awkward position of her head is what I'd be cautious about. Like PP said, she could develop a flat spot and her neck muscles could tighten. There are some exercises I found online to help. (Google)
My daughter is 5 weeks old and from the beginning her head did this while in the mamaroo. I think it's a combination of babies having little control of their necks combined with the angle the mamaroo sits at. Even when you recline it back, it's still at an upward angle. My daughter also tends to slide down in the seat which makes the head problem worse. Now that she's bigger ajd has better neck control her head doesn't do that anymore, or at least not as bad.
You could try wedging a receiving blanket under the side of the head that drops down as long as you'll be in the room with LO to monitor. 4moms also makes an infant insert for the mamaroo you could look into that looks like it offers head support.
Re: Sleeping in mamaroo
Technically they aren't supposed to sleep in those things but I had a Rock 'n' Play (same idea as a mamaroo) and let my LO sleep in it until she was 4 months old.
Does your LO always sleep with her head at that angle? My daughter had a tight neck (torticollis) so she developed a flat spot from sleeping at the same angle all the time. Just some thoughts.
I say let her nap in it.
You could try wedging a receiving blanket under the side of the head that drops down as long as you'll be in the room with LO to monitor. 4moms also makes an infant insert for the mamaroo you could look into that looks like it offers head support.