Dimmer as backup to Thermostat?

[ctrlfreq]

With the Thermostat at full output, it should be able to achieve the target temp (92) while at the same it will be capped at 95 deg by dimmer.

Don't do it. If you want a backup, keep the rheostat on hand in case the thermostat fails, don't try putting it inline.

As for why, a thermostat is just an electrically controlled rheostat, and as such, it's internal circuitry is only certified to operate at a specific voltage and amperage. By under-volting that circuitry, you can potentially damage it, and leave it incapable of controlling the rheostat, leading to possible fire hazards and increased risk to the animals being heated. Beyond that, you would be trying to keep a steady temperature based on a balancing act, and a slight change on either side (ie, bump the dimmer by accident) could lead to dangerously high temperatures.