We will be driving from Ascoli Piceno to Venice in May and staying in Venice (Dorsoduro) for several days. Would it be more convenient to drop the car at the Venice airport or at Mestre?
TIA.

Neither.
Unless you are using an obscure rental company, Piazzale Roma is more convenient if you are taking a Vaporetto to Dorsoduro.

Piazzale Roma for sure as greg says.

Ditto

My experience is 5+ years old, but when we arrived at the rental car drop off at Piazzale Roma on a Saturday (driving in from Montepulciano) the place was jammed. We were in a line for at least 45 minutes just to enter the parking garage, where you do the drop off. Perhaps the crowding was due to it being a Saturday (a conventional turnaround day for rentals), but it certainly took longer than we anticipated. Even so, Piazzale Roma beats Maestre and the airport.

Thanks to all for good help.