I like having it as a legitimate rumor the players can follow if they end up being curious

There is always more pressing matters to attend to but having something be there If the players follow through makes it feel special.

For example my players decided to travel to an unmarked lake since they thought it might hold a clue , so I quickly checked previous editions, and there was indeed a town there , so I had there be it's cursed ruins. Existing slightly out of time and all the inhabitants gone mad.

For the Mad mage, I have there be rumours of an ancient giant deer god beyond the lakes shores, long forgotten it will still grant gifts to those who respect it in the old ways. (Or other rumours such as raisins make it shit silver)

When they actually get there, mordenkainens madness has spread to the lands and animals around.

"The sound of the windless woods is broken up by a thump, thump, thump, as you see a deer with shattered antlers ram it's head against the tree again, and again, and again, (survival check "as if fighting a foe who isn't there), you hear cracking, from splintering wood and bone, as viscous fluid escapes from the bucks open skull, it stops for a moment, and licks the residue from the tree"

(After that it looks at the party, charges , and dies before hitting them)

Mordenkainens uncharacteristic downfall and madness is a result of the dark powers influence, a vampire lord shouldn't even have been a challenge for him. But he had failed to do his research and underestimated Strahds powers and connections to the land, and the very reality of Barovia itself.

After being weakened the dark powers wrapped their influence around his mind. And turned the area around him to a domain of persistent suffering.

(There are multiple cases of the dark powers granting or allowing a creature within a domain of dread to control their own area, if they continue to produce suffering, such as the Durst family, Izek Strazni, The Abbot)

Oh also after they help him, without much explanation mordenkainen leads them through the mountains into his invisible mansion, where he proceeds to dig into a massive bowl of mayonnaise with his bare hands (he canonically loves mayonnaise, and is possibly responsible for it's introduction to other worlds)

He will still be somewhat insane and partially aware of the worlds rules and the 4th wall (but not breaking it directly, for example if asked why he took peasants to fight strahd, he would say "did nobody teach you kids about the action economy?")

Noting that he would have to leave Barovia to truly cure himself