The title text is supported, but only devices with mouse support. When you hover it will show.
(It’s not supported on touch devices, but it’s also much less clear what should be done there.)
Even W3C says
Warning! Relying on the title attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g. requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).
I could support it in the image preview caption, but it would still often not be visible for other users (such as mobile lemmy-ui users using any browser). So in general, follow w3c, use alt text instead :)
It’s planned