No solder paste was applied on Thermal Pad for AGND. What would this do?

To quote the datasheet:

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The large thermal pad underneath the component must be connected to the system ground...

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Note the use of the word 'must'. However, it doesn't discuss whether the device relies on this as an electrical connection to the PCB. It goes onto say 'This provides the path for heat dissipation from the converter'.

I would suggest that, providing the device isn't getting hot, it'll continue to regulate well. As it heats up it's performance will suffer and there is a risk of the part failing. Hence, the need to remove any heat generated by the device from it. Worst case it may fail in a way that takes the rest of the circuit with it...

You could bond a heatsink to the top of the package. This would help dissipate any heat generated by it.

Regards,

Alex