Which 90mm are you referring to? I can tell you my little f/2.8 4-element Tele-Elmarit weighs less than an A7.
One of the older Summicrons? Still not
that heavy. The latest M APO: ~475g. I've had my A7r attached to a tripod with a Zeiss 200/3.5—a hefty lens for its focal length & speed—mounted. No problem with the lens mount handling the weight, though I wouldn't do this with anything physically longer. The main issue would be with longer lenses where much of the mass is located towards the front elements…I've found that's what can torque a lens mount.
-Dave-