Can I use TRIAC dimmer output AC rectify to DC
You are free to rectify the output of the TRIAC.  You should of course ensure that whatever circuits the TRIAC is supplying can accept the dc output. You don't actually say what it is you intend to drive.

To give one real-world example, rectifying TRIAC output has often been done with universal motor controllers.  This can reduce the size and weight of the motor, and decrease wear.  Torque ripple can be reduced, as well as low frequency noise.  This is due to the reduced rms and peak to peak current.