The implication is that they (well.. we) can't do what's necessary in a low-effort scenario I'd say.
Kind of similar to calling someone a mouthbreather
Not really. It's really targeted at not being resilient when facing adversities. A Warmduscher can't bear having to feel cold water, so he probably can't take any other minor lack of comfort as well. Typical examples are people complaining about it being too windy or rainy when unexpected inclement weather hits during an outdoor activity. It can also be used to jab on people who probably they "can only sleep in their home in their own bed" and suffer greatly from such trivial discomforts.
The irony is that when people are working at their hardest they often need to breathe through their mouths in order to get enough air into their lungs.
