Please don't take this the wrong way, but this is your first baby right?
I'm presuming that as your ds has very specific naps, he is in a routine? Ime I have found that this is something which lots of people have very different issues on, and people who have never had routines for their own babies find them very hard to deal with, because they find them too rigid/they can't understand the baby's need to be fed/have a sleep at certain times because their own babies never did it that way and they were all fine.
I would say that it's not so much that your mil thinks your ds naps too much, but that she finds it hard to fit in with the idea that "he needs his nap now," when perhaps she has other things in mind. But as a new mum, you see this as undermining your parenting, and as a new mum, anything said contrary to the way you're doing something can be viewed as a criticism of the way you're doing things.
My own ILs had exactly the same issues with the way we were bringing up ds. Except it got a lot nastier than this, with me being told by Fil that the way I was bringing him up was harmful to him and my sil telling dh that it was our fault that ds was hungry because we always fed him at certain times .
At the end of the day, it's not a personality clash just more a clash of ideas. And in reality the whole routine thing won't last for ever, within the next six months he will probably have dropped two naps and be on only one nap a day.
So I would say that it's a bit much to get so annoyed over something which is really only going to be a very small part of your lives.