First of there is a medical condition called hypermobility .
So yes to much mobility is not a good thing.
Regarding the effects of stretching on strength I found these papers:
"Maximal strength decreased after static stretching (213.2 6 36.1 to 184.6 6 28.9 kg), but it was unaffected by ballistic stretching (208.4 6 34.8 kg).
In addition, static ...