Hmmmm.
First, Schonbrun is a great place to spend an evening listening to music. Is it the premier classical musical destination in the city. No, but if you understand how musicians operate, expect the orchestra to include many talented professionals who otherwise also perform at other "more serious" venues. Thats how it works. Gigs are not exclusive. This will be a night where you can relax and enjoy the music without feeling like you need to be dressed to the nines and judged for how good your seat is.
And (our families and my wife are professional musicians and have made careers teaching, performing, conducting, writing, publishing music) lets be really serious and point out that Strauss is not serious music. And Mozart, for all he is and was, he played at Schonbrun so the context of these performances at Schonbrun is 100% correct. "Mozart's in the closet, let him out, let him out, let him out, as we say."
As a classical music geek, I do not think a performance should be labeled as non-serious if it does not measure to some unclear personalized standard. As someone who has been to the Opera, Musikverein and Konzerthaus (presumably where the "serious" music is performed) I can assure you that you will have a wonderful and memorable evening at Schonbrun.
Go to your concert, enjoy it, feel totally safe on the subways both coming and going.