Can we use 125VAC 20A switch in place of 12VDC 20A switch?
With the exact same 20A current rating? Almost definitely not.

The right answer is always going to depend heavily on the switch in question. In industrial applications I have seen big, heavy switches that actually had the same listed rating for AC and DC applications. From a design standpoint, that is accomplished by designing a switch that can withstand extinguishing a DC arc, and be sufficiently insulated to handle AC voltages. This is a highly unusual case, as it means the switch is significantly more expensive than it needed to be for either case.

Switches frequently have both AC and DC ratings, but in the more common case, the DC current rating will be significantly lower than the AC one. DC arcs take longer to extinguish and cause more wear on the contacts. In a 20 amp application, I expect that an AC switch with the same rating would fail quickly, and badly.