It is, yes! On the jam homepage, there's the following text under the "Submitting" header:
Game files are not locked for the first 48 hours of the rating period. This is for fixing bugs and adding alternative platform support. Don't use this feature to otherwise update your game. After the rating period is over, if you wish to continue working on your game, please keep the original jam version as an alternative download.
It's essentially done on a honour system, which is very permissive, but hopefully people understand that there's nothing to be gained from winning the jam through disingenuous means. They also reinforced this point in the deadline announcement on their Discord server. If you've got some bugs you want to iron out, you still have time!