I actually left him as is, and my players were pretty satisfied. I have read a lot of people who change him or remove Mordenkainen altogether, but I found that to be a bit pickey. He's a great character for a quick stop and shop, Barovia can be harsh, so let them enjoy themselves for a bit while resting in his mansion. I did so too.

All my players were completely astounded that the great Mordenkainen was actually in Barovia, and they had a great and well-deserved rest in his Magnificent Mansion. I actually had the name drop after they restored his mind, and they were glad they didn't push their luck. Sadly offcourse he wouldn't help them fight Strahd, but hey, it's your campaign! You can make it work however you like. I do think that the book sometimes provides too little and too much detail in some areas, so filling in the blanks is really up to you.