Yes, a particle dust mask will help. If is necessary is another matter. Emptying the tumbler outside is a first step, standing upwind another. Pouring it out slowly also helps,
If you feel safer then by all means wear one, also wear disposable gloves and a long sleeve shirt to avoid dust on your hands and arms
Always wash hands, arms and face after handling any hazardous materials, Wear a hat and wash cloths separate from the family laundry
I used to worked with really hazardous stuff and was responsible for worker safety and MSDS of all chemicals
For tumbling brass, I am not that concerned as I do it outside, am careful and the exposure is very minimal and infrequently. Now for people that do hazardous things all day on a regular basis, then it's different,
Shooting at a indoor range that allows lead ammo (primers) will give you more exposure in a short time than tumbling some brass in a closed lid tumbler.
If you buy dust masks, buy the right kind and GOOD ones, 3M is good,
3M
Recommended Application Bagging, Agricultural Production, Processing of Minerals, Cleaning, Dry Chemical Handling, Grinding, Petrochemical Manufacturing, Airborne Biological Particles, Textile Operations, Petrochemical, Sanding, Pharmaceutical Manufacturing, Assembly and Mechanical, Foundry Operations