Is there any problem for allowing this mod to download skins automatically and resolve on cachedImages?

I'm not sure what you are talking about

It is already doing that except, it does not store them, it keeps them in memory until they are no longer used in game for 15 seconds.
Providing PNG for players through the cachedImages folder is really just a way for situations like 1) no internet access; 2) internet access is really unstable; 3) your ISP is acting funky; 4) testing during development; 5) change skins between recordings fastly.
By default, it uses Mojang's and Crafatar's skin server, only a real account with skin access to Mojang's service will be able to have skin in this way.
You can however, use the custom skin server option to make it uses a skin server you setup.

im suggesting that, rather than explicitly providing the PNG for some player name. Automatically resolve player skin when logged. If skin exists (appeasrs on some skin repository) download the PNG and store it con cachedImages/skins.. etc

fair enough