Interfacing 3.3V SD card MISO output to ATmega328P
Are you sure your grounds are all connected? The scope trace looks like something is floating.

You are doing it wrong with the resistive divider. Dividers are affected by the load, and that's probably what is happening here.

Unfortunately the MCU on the Arduino doesn't support open drain outputs in SPI mode. You could implement bit-banging SPI with an open-drain output and then simply pull the MOSI and SCLK lines up to 3.3V. Alternatively use a dedicated level shifting IC or if it's just a hobby project a series resistor will probably be okay. Actually you could use a Zener diode too.

In short, your potential divider is most likely the problem.