The ability to make N/R decks on Omega would be awesome. The ability to filter by that rarity would be really nice, and I think would be the only non-Master Duel app that could do it; at least I have not seen others.
There is already a Common Charity filter on Omega. We do not store quantities of rarities in Omega. You have to understand all cards are free in Omega. In master duel, each user has their own unique database of cards because they have to acquire them. In Omega, you can choose what cards in your deck you want to apply rarities. We made our own rarities ourselves. These are not MD rarities and they never will be.
So to ask your question to filter rarity makes no sense in a game where all cards are free and where any card can be any rarity based on what the user wants.
Looking at it from another direction, why would it be important to do this? You an already build decks in master duel with cards that you do not own. You can also search In Master duel with the rarity filters. Omega adding this in as a “feature” would not benefit anyone as master duel format is specific to master duel, and the limitations of dust and card inventory is also something that is controlled by master duel.