No. They're actually quite comfortable. Not much difference between them and the stationary bed. Same mattress in the ones we've stayed. (Pop Century, for example.)
If you are talking about the new pull down queen size murphy beds (POP, AllStars ...) they are the same exact mattress as the other bed and there is no difference.
If you are talking the small tween sized murphy bed (POR, CBR ...) that would be a smaller than a twin, thinner than a mattress bunk style murphy.
Depends on bed / resort / style / size of mattress / type of mattress / how old it is.