Ethereum

How do people feel about onchain contracts including an optional fee that can be bolted on by frontends? Primarily to incentivize more third-party frontends to be developed.

Are there better ways to do this?

A doodler and computerer. I like permissive/permissionless open source, smart contracts, p2p systems, room-scale VR, and NixOS.

Currently: WhatsABI

If you are using AA, which you probably should anyway for consumer crypto, then your frontend could simply take a fee from any deposit amount or whatever token balance is managed across all of those frontend specific actions. I don't think this has to happen on contract level, because the setup and complexity is horrible.

This is the alt account of Sister Hazel. Writings at https://powerlaw.systems. Building https://testnet.uvio.network. 

Agreed. Though this assumes a defi application.

What about utility applications, for example things like updating ENS fields?

If we talk about infrastructure automation, then I would assume that this is simply part of the mechanism design, and then your answer is "it depends". Probably a case by case solution for which you implement your own registry and onboarding flow. Devil's in the detail's here I guess. I have never seen a great pattern for this.