It most certainly will be too warm. My first sleeping bag was a 0 degree quallofill bag, which was originally bought for shoulder season camping where min temps would be ~15-20C. I also used it when car camping on kayak trips during the spring/summer. I experienced the same min summer temps you expect, and sweated profusely throughout the night. Even when the bag was unzipped and used as a blanket I still was way too warm.

I though it was due to my metabolism (I am usually a warm sleeper), but when I got a new 3 season bag more closely matched to the temps I encountered, I didn't sweat nearly as much as I used to. In addition, I made myself a down quilt, for those 50C summer nights, which can also be thrown over my 25F bag to extend its range into the teens.