The radios and antennas on both the aircraft and for ATC are not only of much higher quality but are also positioned as perfectly as possible so as to maximize Tx/Rx performance.
All that to say, the pilots and ATC nearly always experience more clear comms than the general public has access to.
From my experience as a small plane pilot, radio comms is almost always quite clear. The antennas and radios are aimed “at the planes” not at receivers on the ground, after all.
A Cessna 152 is not an airliner, obviously, but I would think audio quality shouldn't differ all that much between them.
The 152 could have had a 40 year old radio or a crappy antenna. Who knows. As you pointed out - that 152 is subject to a lot less maintenance scrutiny than commercial airliners are.
The other thing that plays in here is that ATC language is very highly standardized. In any specific phase of flight, the pilot will generally know what to expect next from ATC. This allows them to discern instructions (and read them back for verification) even if the signal is weak. In situations like these, ATC may seem completely unintelligible to people not used to ATC language and patterns.
I'm a pilot and even on very old planes like piper cub where the radio is just a desk radio added in the cockpit, the audio is much clearer. And with something like Bose A1 (noise canceling headset, A20 and A30 now), you hear it very well.
Most the time you hear ATC perfectly as they have very large antenna. ATC will hear you worse because of the noise around you, but with good mic it is still quite OK. They have large receiver too so you can be far. The worst audio is always plane to plane, there are some situations where your are far from a plane and only hear the ATC side of the conversation. But even if those situations, you can ask ATC to relay a message. It is very rare as you hardly have any reason to talk with a plane that is 100km away on the other side of a mountain.