It technically can't be zero or your machine could not request anything. Also your machine is sending acknowledgement messages in response to downloaded data so if it was really low it would affect the download speed also. It should cause web pages to load very slowly especially ones that have lots of complexity if the upload is actually low.
Your cable modem should have something that shows the upload power levels. Those numbers should be below 50dbvm.
In many cases cold should actually help, metal contract so physical connections should be slightly tighter. Some electronics has restrictions on low temperature operation but I doubt the inside of your house where your cable modem is located is that cold.
If it has a impact...