Crested Geckos do actually benefit from a higher temperature than most care sheets will state. There are many owners now that offer a ceramic heater (On a stat) at around 29 Celsius. Those doing this find that their geckos seek the heat more and are provided with a much better thermogradient allowing them to thermoregulate more sufficiently.
A heat mat is a contact heater, it pretty much only heats what it's touching. Good for under a tank to heat substrate, but on the side of one it will do little more than heat the glass rather than the ambient temperature of the vivarium/terrarium. Plus a 4 Watt mat would hardly heat the glass.
That's not to say Crested Geckos do not live and survive at room temp, just that many now are going in the direction of offering their geckos more than the bare minimum. Let the gecko decide.
Ceramic set can set you back between £10-£15 and £30-40 for a decent thermostat.
If you have any concerns, doubts or worries about your animals health always consult a qualified veterinarian (with the necessary experience).
Nobody can properly diagnose an animal based on descriptions and photos. Especially when those animals, such as reptiles, have a small list of visible symptoms than can be found across a large range of problems.
Always see a vet. Don't let strangers on the internet make you do something you'll regret. They won't take responsibility.