As you climb the air gets colder so the speed of sound gets slower, So you will get closer to reaching crit mach at a constant TAS . Your TAS will also increase if you are flying a constant IAS as the air is getting less dense so you can reach crit mach that way.
Low speed stall does not increase for IAS, it only increases for TAS but that has nothing to do with mach speed, it is simply the air getting thin.
So to repeat; As you climb the TAS stall speed increases until it becomes the same speed as the critical mach speed. The decreasing air temperature speeds up the process but it would happen anyway as the air density decreases with increasing altitude.
Hope that is clear.