How do I customize my mini-profile?

Unfortunately, you can't. Fortunately, you can add pretty much whatever you want to your signature instead.

If you are a seed trader or such, it is recommended that you put a link to your profile post in your signature, whatever else is in it.

Links are not automatically functional. You need to use HTML for that. Here's the code for my current signature:

Location: SW Idaho, USA<br>
Climate: <a href="https://www.mindat.org/climate-BSk.html">BSk</a><br>
USDA hardiness zone: 6<br>
Elevation: 2,260 feet<br>
<a href="/Raifu-td44.html">Profile post</a>