So I have a few questions:

Has it ever worked correctly - is this a new problem?
Was this recently installed / changed?

A bad dip tube would result in the water temp at the top of the tank getting up to the correct temperature, but it would be diluted with cold water as soon as you turn on the faucet.

A digital thermometer (about $10) applied to the hot water nipple at the top of the tank without running any water will tell you if it's getting up to temp. Apply the tip of the thermometer against the nipple as close to the tank as possible and wrap with a rag or other insulation to get a good reading.

A bad thermostat on the gas control will cause the top of the tank to not come up to temp - and/or to overheat. If that's t he case, you might look at replacing the tank completely.

Do make sure this tank wasn't hooked up backwards (hot and cold reversed). doing that would cause the water leaving the tank to be cool all the time because it's drawing off the bottom of the tank. Your thermometer at the top of the tank would be hot when the thermostat has been satisfied and no water is being used and would cool to incoming water temp immediately when water was run.

Here is a link that might be useful: Digital thermometer

Jake covered it. I can only add that if the problem winds up to be the t-stat/gas valve,don't even consider repairing or changing it,buy a new heater.