Plugging a mono connector into a stereo output should be avoided as it will connect together the outputs of two distinct audio amplifiers. Not good.
Some of the Icoms used a TRS (3 conductor jack) even though they only deliver a single mono signal. If you plug a mono plug into those, the ring and sleeve will short the audio out signal to ground and you will hear nothing or super low volume. The jacks are wired with the single audio signal on both the tip and ring terminals. I forget which Icom I found like this but I think it was the R7000, R71 or maybe the R9000. Those are the only older Icoms I have and one of them was setup like that.
I've never hurt a radio yet by shorting the audio output but of course I start with open squelch and no volume so I can bring it up slowly if I'm unsure of the jack type and wiring. If I do not hear noise right away, I assume it is shorted and I'll grab an adapter to test with. I suppose you may harm the audio output device if you left it in a shorted condition with the volume turned up for any length of time greater than a few seconds or so.
I assume the OP is referring to the IC-R75 from his signature line info. The OP did not specify the actual model.
According to the service manual, the R75 has a standard Mono 1/8th inch external speaker jack so a stereo or mono plug will work but using stereo speakers will only produce an output on one channel unless you use a mono plug to stereo jack adapter.
The R75 also has a 1/4 inch jack on the front panel that is a stereo type setup to drive two speakers in a stereo headset. It is volume limited by resistors though but would work fine with amplified computer speakers.
I've never used the front panel jack on mine.