I wouldn’t say mlem is similar to Apollo. At least not yet. It’s in a very early state and is missing a lot of essential features. You can basically read threads and that’s about it. Posting and replying is super rudimentary, you can’t see your own profile and notifications are just a placeholder.
I’m sure it will be great in a few months but right now, the web version is your best choice on iOS.
The last version added a “Messages” tab but so far it doesn’t have any content.