Is it bad to stack rackmount servers?

Not really. I've got 4 Dell PowerEdge servers stacked one on top of another right now. There's really no risk of collapsing the bottom servers from the weight of the top servers and there's really no risk of overheating, as the servers draw cool air in the front and exhaust hot air out the rear.

The risk is that the platform you're putting them on could collapse if it can't bear the weight of the servers. The only other issue, as jscott pointed out, is the inability to access the innards without moving them off of each other.

If they are just a couple servers and if they are 1U, stacking them on top of each other should do them no harm at all.

Of course, it's quite a less-than-optimal placement... but if it's temporary, I see no problem in doing that.

From a mechanical engineering standpoint, I'll add that you want to make sure the server on the bottom has its bottom surface supported as evenly as possible. This would normally mean putting it on a sturdy flat table or countertop that extends past both long edges of the case. And if its front panel projects downward even slightly below the bottom of the case, position it so that edge of the panel just overhangs the edge of the table.