This was a great entry especially given that this is your first game in Unity! You've added so many things like the boss relocating, laser beams, powerups, different boss attacks, and not to mention all of the other assets as well so this is truly impressive!! 

My feedback would be the following:

1) The game is a bit hard to play as moving backwards is relative to where the bird is facing so i have to think a little bit before moving to where i want to go. 

2) The acceleration of the bird from not moving to moving state is quite slow and so it felt a little cumbersome to stop at all but that may be what you were going for given the jam theme

3) The music is a nice listen! Not sure if this is the feel you were going for but when I die, it is strikingly a different feel then the atmosphere of the game in general. It gave me a jump when i died! 

4) I had to zoom out of my window screen to see the game in full. I have attached a picture below. The resolution of the game must be super high or unity's webgl build exporting may be need some tweaking. (thats at 25% zoom)

Great entry again!! I hope you have time to develop it more!!!