Forward converter with RCD clamp
In case of a Flyback converter, the RCD clamp is supposed to dissipate
  only the energy stored in the leakage inductance.


Correct.


  However, in case of a Forward converter it would dissipate the energy
  from the magnetizing inductance


Also correct but, there is more than an order of magnitude difference between the energy exclusively stored in the primary of the flyback transformer (for use with the secondary) AND the very much smaller level of energy stored in the magnetization inductance of a forward converter. There are many more primary turns comparing apples with apples.

For a forward converter, in the first part of its switching cycle, conventional transformer operation occurs and the primary current is "mainly" the load current from the secondary (referred to the primary via the turns ratio). The actual primary "load" current is very much greater than the magnetization current and hence, when the 2nd part of the switching cycle commences, there is a very much smaller amount of energy to dissipate than that transferred through to the secondary.

That amount of energy is similar to the amount of energy needed to be got-rid-of in a conventional flyback converter due to leakage inductance. The forward converter does this by having about 4 times the number of primary/secondary turns compared to a flyback transformer. 4 times the turns means 16 times the inductance and, for the same switching frequency and primary DC voltage, the magnetization current is 16 times smaller.


  But isn't that power dissipation so much that it makes it
  unacceptable?


This means that the primary magnetization energy is one-sixteenth of that for the equivalent flyback transformer i.e. not in the same ballpark.

Of course, some flyback transformers will run with less than 4 times the turns than the equivalent flyback transformer but, the general trend is that the forward converter uses several times the number of primary turns compared to the flyback transformer.