Not usually. They likely will settle down, and the nymph/s will eat what it wants at their leisure until the fruit flies die (usually a couple of days). If you've put so many in that they seem to be stressing the mantis continuously, I would remove some of them. If a mantis cannot even sit and rest in peace for stretches of time because of too many feeders in with it, it can become stressed to the point of illness or death. Way too many fruit flies in the enclosure when a mantis is molting can also interfere with it molting properly. But I wouldn't concern yourself with that worry too much.
There would have to be
lots of extra flies in there to cause the risks I previously mentioned. I routinely put in many more than I know they can eat, and it works out fine. I just thought I'd mention that you don't want to go hog wild and put
tons in there!