imo a core problem is that perceptually, an app is usually tied software (the mastodon app) instead of network (the fediverse app) or server (the indiewebsocial app)
Mo-Me is leaning into this dynamic. For #MediaLiberationDay, it was very useful to be able to point people to Mo-Me to get started (as oppose to "start by picking an instance" or "start on .social"). And I wouldn't call it a "paddling pool" since it's got pretty rich functionality and is fully federated, but it's definitely an example of what I think of as a "starter instance".
One of the challenges with the app focus though is it's likely to reinforce the default tendency on flagship instances. If I want to use Piefed-the-app, it's natural to sign up on piefed.social; even though they're putting a lot of work into helping people choose an instance when they sign up (and kudos to them for doing it!), why not just sign up on piefed.social? It's hard to answer that without talking about instances and why they matter!
