Some people wear the latest tennis fashions, and other people wear just what they feel like wearing. I used to belong to a tennis club that also had a big gym and people would come play in workout wear.
And to be honest in beginner's classes, I've seen people wearing just regular shorts because they don't have any idea what they are supposed to be wearing.
Wear what you are comfortable in - that's what is most important.
they are fine. i've worn shorter shorts for tennis and more outrageous outfits than you could imagine. i got weird looks from some people who are 'traditionalists' but everyone thought i was a visionary