The problem with dimmers in general, is it makes people think they can get any bulb they want and just dim it until it's the temperature they need. This is not ok.
Your basking bulb provides a couple important benefits to your dragon.
Heat - thats obvious
Something thats not so obvious is a tool to help your dragons eyes dilate so they re protected from the uvb rays.
So you need your basking bulb to be putting out a good amount of lumens too. And if you dim it too much, you lose out on this important aspect.
You have a couple options based on your setup:
You can try a lower wattage bulb. Custom built tanks hold heat much better. I use a 50w in mine.
Or, you can change your bulb fixture. Your fixture right now is forcing the bulb to be really close to your basking surface. If you switched out that fixture for something like this:
your bulb could be further away. You could even make a taller basking surface if you wanted.
-Brandon