Agreed. In theory - could, in practice - not a good idea. Rectify your AC signal and then dim the DC power.
However PWM does not work for all applications so it depends on what you connect to your DC load (trying to lower the voltage with your TRIAC before rectifying likely won't work either for those applications). You can't use PWM on brushless pumps and certain LED modules (those that come with constant current driver rather than current limiting resistors).