Yes they are. They have more HP and deal more damage, but it isn't TOO strong. I have found a Ghoul, Quara, Dromedary and Souleater fiendish, and nothing that can hurt me. I can't tell how this can work for a creature that is already strong, if they can make things harder in a box, for example. But, overall, fiendish creatures are a bit stronger than normal ones.
Also, when using the new spell Find Fiend, if you have unlocked the creature bestiary, the spell shows you the difficult, so you can go prepared to face the monster.
From the teaser :
The first one is dust. In order to get dust, which is non-tradeable, players have to kill influenced monsters. These creatures have a low chance to spawn in place of a regular monster and are stronger versions of regular monsters. They can carry one to five stacks which gives them increased damage and maximum health – the more they have, the stronger they are, and the higher the potential to yield more dust. How many stacks a creature has is indicated below their name in the game window. While a player can only carry 100 dust at first, this limit can be increased to a maximum of 225 by spending dust.
An influenced squid warden...
... and a fiendish dragon.
In addition to influenced monsters, there are also fiendish monsters. These are even stronger creatures, which do however also give more dust, as well as a new tradable resource called slivers. 1