iMac 2019 fans rather loud (and at 2850rpm) when only 30% CPU - is this normal?
Without hearing the actual “noise” it’s pretty subjective as to what the actual level is.

However, I did notice that you’re running at least 4 instances of yes that you’ve probably detached from the user so what you’re seeing in that 3rd party “analyzer” is incorrect.  Activity monitor will give you a better view.  

That said, yes is designed to max out your CPU and, as stated, you have at least four instances running.  Your CPU fans are going to turn on especially when the CPU is running at 90 degrees (the thermal cut off is usually around 100 depending on the CPU) under load.

So, what you’re seeing is perfectly normal and resetting the SMC won’t “fix” it because nothing is broken.  Why?  The fans come on based on temperature not on load.  And, temperature is more than just CPU temperature.  There are several temp sensors, even ones that measure the ambient air that go into calculating when and at what speed the fans spin up.  So trying to evaluate a malfunction of the SMC or a problem with the fans based on one factor is incorrect methodology.  

So, is this expected for Mac computers?  

Yes.  It’s no secret Apple has poor cooling and uses undersized heat sinks/pipes, and even uses (IMO) sub-standard thermal paste between the conductive surfaces.  It’s a tradeoff between the super elegant and ultra thin design and efficient thermal performance.

Your Mac is running as expected.