According to this wikia page on Dice Poker, higher level players do have better "luck":
This may be a 'game of chance' in theory, but in reality some players are inherently 'luckier' than others, including yourself. What this means is that as Geralt moves on to progressively more advanced players, the system that calculates the dice values is modified to favour the NPC, with the NPC having a greater probability of getting a better hand than the player. At higher levels this can be particularly frustrating, with Geralt's opponent easily cranking out great hands with each throw when he himself has trouble getting three-of-a-kind.
So, basically, yeah, the computer is cheating.
There seems to be a possible way to game the system right back, though. From the same wikia page as above:
The initial bet seems to determine the overall 'luck' of the game. A low first bet dramatically improving the chances that you will beat your opponent. This is unconfirmed mathematically, but has been anecdotally observed many times. (This is as close to being confirmed as is possible without empirical evidence, having played twenty games of poker dice, ten where the initial bet was the largest possible one, and ten where it was the lowest, the statistics speak for themselves: Of the ten first games, eight were lost and only two won. The remaining ten (where the initial bet was the smallest one), resulted in nine wins and only one loss. Furthermore, the chance of getting an initial combination of matching dice seems to improve drastically when opting for the smaller initial bet. Naturally, this does not guarantee a win, as mentioned above, the game is still about chance, this little trick does however seem to improve your chances of monetary gain. This was tested on the Enhanced edition of 'The Witcher'.)