So don't feel bad you can't help out much. I mean I'm still recovering from my stroke a couple of years back, and get exhausted easily, so this was all I could do ;-)
On that subject, you're going to be tired and stressed, but mum is likely to be even more tired and stressed, especially if she's feeding. When you have the energy yourself, make the time, even if it's only for five minutes, to pamper her a little, or encourage her to pamper herself.
Some personal examples.
The shower I mentioned in the previous Useful Stuff post. I didn't mention that I spent some of that time washing Sharon in the shower. Nothing erotic, or sexy, just a nice straight-forward wash - carefully avoiding tender parts like her breasts - just so someone was looking after her needs and making her feel special for a change.
The other was Sharon having a fancy for roast duck, but deciding that ordering it for herself alone would be too expensive and a waste. It came to about $26 delivered. I told her that she was working bloody hard with feeding, as well as the shared duties of changing, washing, etc. I knew we could take the money from our shopping budget. Yes, that much less to spend on food, but on the other hand, this was something that she'd really enjoy, in amongst all the hard work involved with the baby. Okay, this wasn't direct pampering from myself, but it was stopping her from denying herself when she didn't need to miss out.
While in Broken Hill, I got Sharon to express some milk and I took a fairly cranky Lex for the night while she took herself out to nice restaurant for a few hours. It let her have a nice meal, and gave her a couple of hours away from Lex. She only did it because I knew she would like and and insisted she go.
It's too easy for a mum to become a martyr, don't let it happen.
Be prepared to run interference for your partner or friend. Sharon's mum totally rocks, I've never seen her annoy or frustrate Sharon, and she doesn't annoy me. However, before she came to visit, I had the conversation with Sharon that if her mum was doing anything that she was finding stressful, to tell me and I'd be bad cop.
I thought of it because the house was (and is) a total disaster area, and Sharon usually gets really stressed about getting the house cleaned up before her mum arrives. I should point out here that Sharon's mum doesn't expect or need us to do this, this is totally down to Shaz. But if Carol started offering to clean up because Sharon's obviously concerned about the state of the house, then suddenly we'd have had to start figuring out the logistics of it - what she can do, where things can go, etc. At the time, if we had had that ability, we'd have been doing it ourselves (when we had the energy - hah! (actually, we are doing it, but in little dribs and drabs, when suits us, which is almost never)), but we're mostly choosing to concentrate on the baby and ourselves. The house can look after itself.
You may need to run interference for other things/people, so do it gladly. Don't make it out to be a problem for you even if you hate doing it. The point is to reduce mum's stress, not have her stressing about how much you hate reducing her stress!
Try to regularly check things like the basket that is used for the baby's dirty clothes, and if it's reasonably full, put on a load of washing. If it's not, put on a load of adult washing. It sounds obvious, but many partners miss these things. Mum likely has way more on her plate and mind than you do, and every small job that she doesn't have to deal, makes her day that much easier.
Also do your best to look after yourself so you can help out when needed. That said, if you're wiped, don't be afraid to ask mum to give you a break for a couple of hours. Think of it this way, you're no good to either of you if you're not coping.
There's a lot more I could put on this list, but I'm tired, so will allow others the chance to contribute ;-)