I don't think you can dim a transformer.
You can dim a transformer, but many dimmers don't like it.. There exist solid state dimmers that are designed to dim transformers, or you could always use a autotransformer.
I've actually used a SCR dimmer to drive an autotransformer, which was used to buck a series string of (9) 12v lamps that were used as footlights on stage for an opera production.
I did have to put an incandecent lamp (ghost load) in parallel with the autotransformer in order to stop the breaker on the SCR dimmer from tripping, but once I did, it worked just fine.
Some types of dimmers are hostile to transformers.. I've seen rainlights (6v PAR36 pinspots) burn up when connected to a dimmer.
