Dimmer is a poor man's way of adjusting your under tank heating pad. It's by far not the most ideal, because dimmer switches do not turn off your UTH if it overheats. Overheating is why you want to get a thermostat as it'll automatically adjust the temperature for you, and will shut the UTH off once it goes over a certain temperature. It pretty much allows you to have peace of mind 24/7, 365 days a year, and you can just leave the UTH running without having to worry about it.
Dimmers will let you manually adjust the UTH, which works as long as you are there reading the temperature, and that you are there when the UTH overheats. But nobody can be there watching their geckos 24/7, 365 days a year, that's why dimmers are just an ok temporary solution. They are not ideal and does not give you that safety net. Are you better off having a dimmer than not having anything at all? Yes definitely, a dimmer at least allows you to tune the temperature up/down. If you don't have the funds to get a thermostat then you don't have the funds to, we all have our budget & limits. But it really is better to have a thermostat.
You can get that Hydrofarm thermostat from amazon for $31, free shipping and no taxes. May sound like a lot of money, but think of the time you save not having to check the floor temperature. Or the brain cells you save not having to worry about the terrarium temperature when you go on vacation or away for a few days. Some things are just worth their money, a thermostat is one of those things in my opinion. Your gecko is going to be alive for a long time, might as well make sure the gecko gets to live a good life, and you get to not worry so much about having to manually adjust temperature all the time.
By the way, now's summer time, temperature can spike during the day. With a dimmer you'll have to manually adjust it when you hit a hot day. With a thermostat, it's all done for you.