Basically, unfortunately we don't support tagging via Discogs, for the reasons Freso mentioned.
It is a daunting idea to start adding all the releases you need to MusicBrainz, so I think it's unrealistic to expect you to jump right in with your time (especially if you have a lot to add).
But if you are really passionate/interested in getting comprehensive tags and data, or would like flexible tags (eg if you're spending a few weeks/months/years! perfectly tagging everything, I would highly recommend Picard so that you can add or change tags across the whole library with a few clicks in future), it should be worth your time and effort.
Also, it might not be the 'official' stance, but I don't always double check information from Discogs. I have way too much to add (all the music), and because they're a marketplace their users value specific data. It's not always perfect, but personally I want to spend 1-2 minutes importing a release, and if someone else has better information they're free to fix it up later - the edit notes clearly show my source, so inconsistencies can be followed up.
Sorry to anyone I've enraged with that admission
To make life easier Discogs often has scans, which is actually the best source (after having something in hand) we could hope for.
Out of curiosity, what genres are you adding that are generally lacking on MB @kema?