I admit, that collecting all the data in the right format is a problem. It takes time and a fair number of steps in a workflow.
Some of the problems arise from very specific local conditions. And these conditions then require local solutions.
Isn't scaling a problem of the target application?
In MP3tag you can set the display of covers to "keep correct ratio" (right-click on a displayed image - this is the bottom option in the context menu, I think).
So MP3tag can show the covers just as they are, if you say so.
MP3tag only modifies the tags of files. MP3tag does not modify the audio part or other complex object containers like graphics.
Mp3tag imports the pictures "as is" - faithfully along the GIGO (garbage in, garbage out) principle.
But you can export the graphics, modify them with a tool as powerful as you prefer it, and then re-import the modified graphics.
Modifiying graphics can require functions as powerful as offered by Photoshop - which is already available - and it costs you dearly. Just to get the proportions right: MP3tag is freeware, adding a (powerful) graphics editor ... might overstretch the bow somehow.
Anyway, I doubt that any graphics editing functions will ever be added to MP3tag. Or do you know whether Photoshop can tag mp3s?
But this is just my opinion and my way of reasoning.
The request for resizing functions goes back to at least 2010:
Re-size embedded artwork