It's definitively cool to see Mode 2 in action, considering it has been rarely experimented with (notable users are Kipernal, me, Von Fahrenheit and the author, naturally) and vertical parallax scrolling is surely one of the simplest implementation of mode 2.
The only thing what bothers me with this submission is the inclusion of an example image or rather, a lack thereof.
Note that if you use a non-vanilla status bar or have it removed, you may want to remove the second hijack in VPatch.asm.
This just crashes my rom when I enter the level...
you've probably inserted a patch that nukes the statusbar, you need the layer 3 tilemap in order for mode 2 to work (unless you change the VRAM address dedicated to layer 3 tilemap but that's another story).
PS: When using this UberASM, the priority of the pipe tiles must be set to 0. Otherwise the pipe will not blind the player and the piranha plants.
That only applies if you use layer 2 foregrounds which makes sense for Mode 3 since you want to use the more detailed layer as a background but not for Mode 2 which uses the classical system of layer 1 foreground and layer 2 background.