Has anyone mentioned the free Audacity? :B Another vote on my part. It's a professional grade audio editor. Even use it at work connected via Dante card from Yamaha mixer to iMac to record 8 tracks simultaneously. Not an audio techie by any means, but have had no learning curve issues using it.
Here is a sample Audacity cassette digitation workflow.
Your cassette to mp3 transfers are obviously not going to sound as good as from a CD, as you are converting analog with fairly low dynamic range source to digital. My guess is that these portable cassette solutions have low grade quality parts, and likely not the best AD converters?
You'll get better quality results if you use the old cassette stereo system player/recorder, if you still own one? Or can pick one up at Goodwill fairly cheap at around $10-15. And use a semi-pro USB encoder such as the
BEHRINGER U-Control Uca202.