We are making our way to Agrigento from Siracuse in a few weeks. We are considering staying at a property in Maddalusa (on the coast just outside of Agrigento). We will not have a car. Is there easy and convenient transport through taxis or Uber to and from Agrigento proper and the Valley of the Temples?

I've had an either a car or used a driver when I visited Agrigento. I imagine you would have to phone a taxi service to get around. Your host or hotel should be able to help with that, but it will likely be pretty expensive. I doubt if t here is Uber around there but I don't know for sure. I stayed very close by last year and without a car it's not all that easy.
Is the beach the attraction of that location? Because it won't be beach weather in April and even if weather is nice, will the beach be groomed/cleaned?

Yes, there are taxis in the area, but you may want to reconsider your plans about renting a car. Getting around that area will be far easier with a car - sites, restaurants, supermarket, or even the beach. You could pick up a rental car in Siracusa and drop if off whereever you are flying out of. Driving outside villages and cities is easy. Roads are good, and gps maps work well. Driving in cities can be hard - between parking, traffic, pedestrian zones, narrow streets built before there were cars, one way streets, etc, but rural / small towns it's not an issue.

I would not count on just finding a taxi with short notice - unless you were at the temples, or in Agrigento for dinner when a restaurant could call one for you. April is shoulder/slow season, so don't expect quick response if you try to find one yourself. It would be good if you had a taxi drivers # to whatsapp or call - get the contact info for one you use early in your stay there to see if they can be your "driver"

There is a bus from Agrigento to San Leone Via Viareggio (every 1 - 2 hrs). From ther it's a 1 - 1 1/2 mile walk up to Maddalusa.