The stock cooler that came with that 3700x should be fine while often the Stealth that comes with the 3600 is pretty anemic the wraith prism that comes with the 3700x is really pretty good on that 65w processor. That doesn't mean it can't be defective or that maybe you have a mounting issue. I would take it off and clean off the thermal paste assuming you have some replacement paste you can use. There have been reports in these forums of some of the coolers having issues with the base heatpipes not being flush as they should be and not making good contact. Or even a fan could be defetive and not running as fast as it should. Even though your 2600 cooler is a lesser cooler you could also just test your old cooler on there to see if by chance it is any better. If it is, it may be a good indicator that you also have a cooler issue. Anyway if you suspect a cooler issue or just a bum CPU and are in the first 30 days exchange it with the retailer and try another. All that being said, yes even a cheap aftermarket cooler would be generally better than what comes with it. I use a Coolermaster 212 evo with my 3700x and it is much better than the stock cooler.
Before doing any of the above I would reset your bios to defaults. Don't change anything and retest. Use AMD ryzen ballanced for your power plan and see what Ryzen Master reports for you temps. Then try the above if that does not help and see where you are now in Ryzen Master.
If you are past your 30 days and need to RMA that cooler and CPU you can start that process here: https://www.amd.com/en/support/kb/warranty-information/rma-form
