I don't know if it would be tacky or not. If the lounge isn't being used much and few people to no one there, I don't see a big issue. The one thing I might have done would be to ask the hotel management if there is an issue with my bringing food into the lounge.

The lounge is usually larger than a hotel room and more open. As such, I can see why you might want to take a meal there rather than eat it in your room.