All My Dell Optiplex 3050 SFF M.2 SSD Slots overheat.

The heat sink should help but I agree that the SFF is a bit air flow restrictive. There are a number of videos on You Tube about modding the case to add a front fan.

As far as I remember Optiplex 3050 SFF has only one M.2 slot for NVMe.

The rest depends on performance of your drive.
If performance on other machines is worse and with Optiplex 3050 SFF is faster - then NVMe delivers higher bandwidth and it's normal to consume/emit/absorb more power.

Generally operating temperature for older (Gen3) Phison chips (NVMe Controller) like E12S (up to 3.4GB/s) and E13T (up to 2.5GB/s) is up to 70 'C. Controller power requirements are up to 4W (E13T) and 8W (E12) {Samsung' ones are roughly the same} therefore it's recommended (on newer Dell models) to install heatsinks on NVMe drive controller (memory chips don't need to be cooled, but it's better not to block area above chips to ensure natural air flow) and you would see thermal pad marked area right under M.2 slot placeholder (I think thermal pad is roughly 3.5-4mm thick, I normally apply several layers of 1mm or 0.5mm}, don't use cheap ones with very low thermal conductivity {below 3 W/mK}, it's better to get something starting from 8 W/mK, non corrosive, non-electrically conductive, non-toxic, etc.) because PCB/motherboard textolit material is considered to be a relatively good natural heat dissipator for low power devices (but it also depends on whether they have other thermally sensitive elements mounted in the same area {underneath NVMe drive area or even on other side of the board}).
Even if it heats up too much - it shouldn't be a problem (at least within warranty window, although it does affect longer-term durability) because drive supposed to thermally throttle down performance if temps reach some upper thresholds, it will slow down operating at lower power mode until it cools down to normal temps.

Thanks for the info on the thermal pads.  I am considering adding a 2nd M.2 drive to an Optiplex 7000, and did wonder why there was a small area beside the 2280 slot for a thermal pad, which is only the size of a 2230 card.

Dell NZ do list a couple of thermal pad options, although neither is the same part no. as the one installed at the factory.  One is for Optiplex SFF up to xx80, so I am guessing that should be close enough.  Weirdly, the 2280 slot has a standoff without a screw and the 2230 a standoff-size screw.  At least I only need to find the screw for the standoff.