I was in Chaoyang park a few days ago (in no way associated with those rapscallion mask-averse foreigners). I didn't notice any signs at the park entrance stating the need to wear a mask, but whilst walking around I heard loudspeaker messages in both Mandarin and English, the English version was ambigiously phrased as, "Please wear a mask when entering the park." This could easily be intepreted as meaning that you are required to wear a mask when at the busy park entrance point but are then free to take it off afterwards (being in the open air and all). I did later see a sign in the central area of the park with the more explicit "Please wear a mask" in both Mandarin and English.
I recall this website delving before into the varying opinions on the efficacy of mask wearing outdoors, 'The WHO doesn't recommend it' yada yada yada. Obviously, even if you're cynical about it's effectiveness, following local regulations is necessary.
As for the xenephobia, it's always been bubbling under, long term expats will recall the anger of local Beijingers in 2008 directed at the pro-Tibet Olympic protests in Europe, with France and Carrefour coming in for particular opproprium, and the anti-Japanese Diao yu island protests in 2012. This is a much more serious, long-lasting crisis that is still (in global terms) in the early stages so don't expect too many nuanced, open-minded views to come to the forefront of local media anytime soon.