I think you are worrying too much about this. I can understand that the reality did not live up to your expectations, which must have been very hard, especially as you had put so much into making the long journey possible. But a lot of modern Mums are like this - the whole bonding thing is plugged really hard and,if she is a bit of a nervous first time Mum, then she is going to take this to heart and perhaps seem a bit OTT.
Please do not forget that to her you are a stranger. You have landed from nowhere in her home for a whole month at a time when she is trying to establish feeding and is feeling overwhelmed with her new responsibilities. The last thing I would have wanted was someone staying for that length of time at this difficult stage, especially if I hardly know them - or indeed do not know them at all. First time Mums are very sensitive indeed to any hint of criticism and need very gentle handling.
If you will forgive me saying so, I think you have already made a significant tactical error by expressing your dissatisfaction (criticism?) to your son behind her back. Not a good foot to get off on.
You have some ground to recover, which involves you accepting this lass as she is and doing everything you can to bolster her confidence and to strengthen the bond between her and your son. This does mean not saying anything at all adverse about her to him - and maybe you could send her an email that is full of positive things that will bolster her confidence: what a lovely time it was; what a beautiful baby; what a devoted Mum she is and how well she is doing; how delighted you are for her and your son etc. You have some ground to make up and I am sure it can be done, but it will involve putting an end to any resentment that you now feel and sending out a positive vibe.
I can see where you are coming from but I can also understand how she must be feeling. The desired end in view is that relationships should be good and every ounce of your effort should go into this when you are in touch with them. Don't forget that you have no rights at all, and that in some ways you need them more than they need you. That is a harsh fact of grandparenting especially from a distance where you are not in a position to offer any help to them.
Like parenting, we get better at being a grandparent as time goes on.