Chosen Solution
I have the same issue on my iPad mini 2. The lines started out as just a small group but then started getting higher and higher. It has been exposed to heat in the glove compartment of my car, parked in the sun.
Those LED LCDs are made from a series of layers. At least one of the layers can be damaged by heat: it wrinkles and the image becomes wavy. If still under warranty, try to have the unit replaced. If not, live with it or take it to an independent but trustworthy repair shop and ask for a quote. If it was me, I’d live with it.
