Mlem doesn’t have this option just yet, but we’re adding it the next beta. Memmy and Arctic have had it for a while.
Adding such an option is very easy to do for iOS apps - Apple provides a simple API for it, so we just have to add a settings toggle. I’m not familiar with how it works in Android development, but it may be more complicated.
Your browser can already do that though
True, but most other Lemmy apps, give you the option to just automatically open the article in Reeder mode. 
Going through old mentions…
Do you have an example?
I think Mlem lets you go directly to reader mode.
Mlem doesn’t have this option just yet, but we’re adding it the next beta. Memmy and Arctic have had it for a while.
Adding such an option is very easy to do for iOS apps - Apple provides a simple API for it, so we just have to add a settings toggle. I’m not familiar with how it works in Android development, but it may be more complicated.