Depends. Birthing is hard on any animal, so it does happen sometimes that a female will rupture something while giving birth, or contract an infection soon after, things like that. Unusual to have lost three right after birth though.
Where did you get the "mummy guppies" from? Were they ones you'd bred yourself, or ones you bought from a store or breeder?
Guppies in general are not nearly as hardy as they used to be. Poor breeding practices and genetics from being mass produced in fish farms abroad has meant they can be quite weak, often come loaded with worms and other diseases, and are also often raised in sea water, making the adjustment to freshwater when shipped to the US and UK hard on them. Sometimes you just have to hope that you manage to get some fry from them, and then the offspring are usually healthier than the adults you bought.
