I answered this more fully in this post right here, but just concisely:

Yes, Tom Cruise was wearing a wig. Thinking about it now, I think it was a partial wig and blended with his own hair somewhat.

Admittedly it wasn’t the “glorious mane of pampered yellow hair” as described in book!IWTV, but I think his performance really made up for any of his physical shortcomings (pun intended, lol), like not being blond enough. 

New thoughts, too:

In fact, he was a brighter blonde as we can see in some stills from what appear to be cut scenes, and they modified his hair and makeup, so that may be a reason why those scenes had to be cut. This one looks oversaturated but you can still tell that his hair is very different from the movie, and his eyebrows are bigger and brighter. Pastier white skin, too.

^I think this would have been the scene Lestat makes Louis watch him take a victim to make sure Louis reeeeeeally wants to vampire. Or it’s the scene Lestat bashes open Louis’s wife’s coffin to show him the decaying corpses of his wife and child to remind him what death actually looks like and it’s hella not fun.

Compare the above to the hair and makeup he had in the movie before the murder attempts: