Multilooking tool job

Is Multilooking a tool only to reduce the speckle in SLC SAR images?

Please have a look at these topics:

So, Multilooking tool makes that pixels of the image become from rectangular to squares, but that helps to reduce the speckle in the interferograms?, because someone told me that the speckle only affects GRD images and no SLC images when I apply Interferometry. So, I am confused.

It can also be applied to interferograms.
But the filtering effect has its limits when the image or interferogram quality is bad.

What data do you use?

I am using SLC (Single Look Complex) data.

Sorry, I meant which sensor.
Can you please provide a screenshot of the interferogram?

CSAR sensor from Sentinel-1