I don't think this would be a great idea, a ratio doesn't help much when discerning who's more deserving, and discerning between users "deserving" is not the goal on the SO sites, from what I can see anyway - every good question deserves a response.
Some people are self learners, who don't actually ask much (though read a lot) but "give back" in other ways (flagging, vote to close, vote on their preferred site, AND on meta). These things cannot be seen or taken into any ratio. IMHO these users are discernible by the fact they actually wrote well constructed questions in the first place, so if you have a choice of two and they're both good, it's likely they're both 'deserving' so just choose one!
The user's rep itself should usually be enough to tell at a glance whether someone is actively participating or "giving back to the community". Someone who answers lots and gets very little of their answers accepted and/or gets some downvotes would skew any ratio anyway - these users may not actually be 'giving back' they may instead be causing the moderators a headache
On the other hand you may just be answering a good question from a user who has just happened on SO(or related) recently has spent some time trying to figure out the best way to get involved, to become an actively participating member and who has then constructed the best question (after making sure it's not a dupe!) their ratio would be pants but if they would be ignored on the basis of that, it could be somewhat detrimental to the future participation of the next generation.. the barrier to entry just rose if you like
I see more downsides to upsides on this one