The thing is, it's about the pram covers, but it's also about so much more.
My mum was like this when I was pregnant, and I also got the "oh but you survived!" Yes, I did survive, but lots of babies didn't due to things that we now know to avoid.
Whenever my mum pulled that line on me, I'd say;
"Years ago, there were no baby car seats, babies were put on the back seat in a Moses basket. Lots survived, but some didn't."
"Years ago, babies cribs were painted in lead paint. Lots survived, but some didn't."
"Years ago, babies were fed solids at 12 weeks. Lots survived, but some didn't."
"Years ago, people smoked around babies because we didn't know the risks of second hand smoke. They may have survived, but many had related issues."
The point is, anything that increases the risk of SIDS is to be avoided at all costs. Put your foot down, because it starts with the covers, and carries on from there.
Your mum needs to realise that you want to do what is best for baby, and that means following current guidelines.
She also needs to realise that they aren't her babies, she had her turn to raise her children, dress them how she wanted, etc. These are your babies, and you get to decide what you want for them.