User import: possible to check 'Notify user of new account'

No, Feeds currently doesn't have this feature.

But I'm open for adding this to Feeds, if you (or someone else) wants to develop it.

How the feature could be implemented in Feeds

It could be made in the following way:

1. Add a new class called UserProcessor, inside the folder src/Feeds/Processor. It should extend \Drupal\feeds\Feeds\Processor\EntityProcessorBase.
2. A new form class called UserProcessorForm would need to be added, inside the folder src/Feeds/Processor/Form. It should extend \Drupal\feeds\Feeds\Processor\Form\DefaultEntityProcessorForm.
3. The UserProcessor class should set the configuration form to this form class (in the annotation).
4. UserProcessorForm would define the option 'Notify user of new account' (machine name 'notify').
5. The configuration option needs to be defined in config/schema/feeds.schema.yml for feeds.processor.user.
6. A QueueWorker plugin needs to be added to take care of sending the notifications.
7. In the EntityProcessorBase class I think we need to add a new method called entitySave(), in there we move the following code from the process() method:
   

   $this->storageController->save($entity);
   

   And then in the method process() instead of initiating the save itself it calls entitySave() instead.

8. In the UserProcessor class, the method entitySave() can be overridden and based on whether or not a new user got created AND if the option 'notify' is enabled, it would queue sending the mail notification.
9. An automated test needs to be implemented to ensure the feature works at intended.

It are quite a lot of steps, but I think that a seasoned developer could implement this.

The feature, outside of Feeds

Alternatively, you could implement sending the user notification in a custom module. For this I think you would need to implement an event subscriber and listen to the event \Drupal\feeds\Event\FeedsEvents::PROCESS_ENTITY_POSTSAVE. Looking at the code, there might be a challenge to find out if the user is new or not. So perhaps a small change in Feeds is needed to add that information.
Implementing hook_user_insert() could also be an option, but then it's harder to know whether or not the user was created by Feeds.

I hope this helps!