I suggest that there are two major reasons,

i) the electrochemical surface area decreases after pressing as Cornel said,

ii) the thin NiO layer on the surface is partially removed and the inner metallic nickel exposes to electrolyte after pressing

If you are using Ni foam as a current collector in an alkaline electrolyte, you should be very careful since the use of Ni foam is tricky. The surface of Ni foam will become nickel hydroxide during the electrochemical measurement in an alkaline electrolyte, and the current contributed from Ni foam will increase gradually to a very high value after hundreds of cycles.

Since current in solids depend mainly on scatering with lattices then the decrease could be most probable. Due to compressing the lattices will dislocated from their posisions and that strongly affects the current density

for more details I suggest to read the following book

I suggest that simply the area of the active/contact surface of the Ni electrode is much lower for the pressed foam.

I agree with Cornel's answer. It seems that after pressing the foam the electrochemical area has been decreased. Ni foam is a very porous structure, so I guess that the porosity would decrease if you press it and so would do the surface area in contact with the solution.

I suggest that there are two major reasons,

i) the electrochemical surface area decreases after pressing as Cornel said,

ii) the thin NiO layer on the surface is partially removed and the inner metallic nickel exposes to electrolyte after pressing

If you are using Ni foam as a current collector in an alkaline electrolyte, you should be very careful since the use of Ni foam is tricky. The surface of Ni foam will become nickel hydroxide during the electrochemical measurement in an alkaline electrolyte, and the current contributed from Ni foam will increase gradually to a very high value after hundreds of cycles.

I agree with Zhao's answer. Nickel foam is a porous structure, the porosity could decrease if you press it, and so the Nickel electrode may much lower for the pressed foam.

By pressing nickel foam the pores area will decrease and the crossectional area of conductor will decrease as the colleagues hinted. T there is also the other effect of increasing the current path length which leads to the increase of the resistance. So, the area of the conductor will decrease and the its length will increase leading to an appreciable increase in the resistance of metal foam when compressed.

Best wishes