It is always a good idea to mention it to your child's doctor.
My now 6yr. daughter, sweated like crazy too when she was a baby at nap time, bedtime and in the car seat.
If the mattress she sleeps on is vinyl/foam (which most are) put some extra cloth between that and the sheets, like a cloth diaper or REAL cotton batting.
My daughter use to sweat so much that even in winter (Ohio winters at that) I wouldn't hardly put anything on her when I took her out anywhere. I just covered the baby carrier with a blanket. Of course, my mother-in-law thought I was horrible for doing that, and I always said, well, she sweats too much to put much on her. I found out one day as I was putting her back in the van, someone was staring at me and I thought oh no, here comes another why don't you have heavy clothes on her, only this mother said don't worry, I did the same with mine for the same reason.
Now she still sweats but not nearly as much. So hopefully it will go away, but do mention something to her doctor.
Also, check the foam/vinyl mattress and put something like cotton in between her and the vinyl, and that will cut back a little on the sweating.