Seems reasonable, especially if your PSU is has a temp-controlled fan that will spin faster as its own temp increases, and/or the components in your PC won't come close to the PSU's max rated load. (e.g. a 400W power supply with a normal non-overclocked CPU rated for a max of 80 or 110W, and no separate graphics card or a moderate one.)
If you almost never run your computer at high load / high power draw (e.g. gaming, video encoding, number crunching) for a long time, you mostly only have to worry about near-idle power draw. Modern desktop PCs may idle at about 15 to 20 W total, but mobo+CPU+SSD full power can be up towards 150 W even for a "consumer" quad-core chip, and beefy GPUs can consume hundreds of Watts on top of that. So make sure you check temps under load unless you only ever do short bursts of load, like compressing a medium-sized file, or compiling for a minute or two. (It takes time for the thermal mass of everything to heat up).
Hearing the PSU fan spool up will be a clue you might be pushing it, temperature-wise, if your PSU fan is temperature-controlled.
Also, I assume you have a separate rear exhaust fan in your case to ensure good airflow over the CPU heat sink and other motherboard components like RAM and voltage regulators, and hard drives / SSDs. Fortunately you can usually monitor temps for many of those components via sensors in the CPU / motherboard, and SMART for hard drives / NVMe, so definitely do that to see how much difference it makes at idle and load.
The more heat the PSU has to get rid of to keep itself cool, the less good an idea this is. E.g. if had a 1000W PSU and actually used a significant fraction of that capacity with some dual GPU setup, at 500W output load with a PSU that's 80% efficient at that load level, you'd be creating 100W of heat inside the PSU box itself and would need some serious airflow to keep temperatures down.
(Check the vendor's specs for temp range, and leave some headroom for dust buildup and hotter days in the future if you have a way of monitoring the PSU temp at all.)
One concern would be if the exhaust air gets deflected forward (instead of out away from the case) and mixes with air that gets pulled in to the front intakes, raising the effective ambient temperature. Your setup (under a desk) might mitigate that some, and there's lots of room air to mix with. (Especially if you have some air movement in the room).
Hot air rises, so that should help it stay away when you just deflect it mostly upwards.
Feeling the exhaust air by hand is a good check for PSU temp, maybe also touching the back grille. Make sure you do this with the PC under high load, preferably higher load than you would normally impose so you have a worst-case idea with some headroom.