I'm not sure why DoorDash should be responsible for disclosing that; DoorDash may not even know what the regular in-person menu prices are in the first place.
The solution to this is to stop allowing delivery services to take a cut of the restaurant's revenue. Instead they should be required to raise their delivery/service fees charged to the customer. Of course, this will likely result in fewer customers, so they don't want to do that, but it's the only honest thing to do.
I get that the delivery services do provide value to the restaurants in many cases, but ultimately the customer pays for it either way, so that cost should be made explicit to the customer.
If it's worth it to me at the price offered, I'll buy it. If it's not, I won't.