The mp3s that I have heard are substandard and I would not bother even trying to seek out such recordings unless the source materials were exceedingly rare.
There are plenty of "grey market" records that compile up old 45s and other cuts. Some of the recordings may be public domain, some are not. Few are probably owned by major corporations but that makes no difference. The original artists and copyright holders see none of the money from those releases (some companies will set aside a small percentage of the profits for any artist who steps forward seeking payment).
Is it any less ethical to download such recordings ("don't bootleg my bootlegs")?
As with any "lossy" compressed data format, there's a trade-off between size and quality.
When someone "rips" a CD and makes MP3 files from the music tracks, they choose what sampling rate to do it at -- the higher the sampling rate, the better the fidelity, but the larger the MP3 file. There's also an issue of using the proper amplitude range -- choose a bad one and the quality will suffer.
A good MP3 of a CD track is very, very good -- indistinguishable from the original unless you're gifted with extremely sensitive hearing. It's also quite large. Unfortunately, a lot of the MP3 files floating around the internet have been poorly made, or intentionally sacrifice quality for size.