Getting Accelerometer values from MPU 6050
Looking at the datasheet of the 328p (page 297), it seems like you're handling the TWINT bit wrong.

By the datasheet, the bit is never cleared by the hardware, which is what you're seeing. It's set by the hardware instead.

By a quick skim over the code, what you could simply do would be inverting the logic around the TWINT - clearing it to make it do stuff, and waiting for it to become set again.