A temperature widget for desktop mode.

[J R.O.C.]

I have one on my windows pc got used to keeping an eye on it just sitting displaying temp in the taskbar

[WarnerCK]

KDE has no shortage of widgets. Right-click on the panel and select "Add Widgets..."

System Monitor Sensor might be the kind of thing that you want, or there's the "Get New Widgets" button if nothing in the default list appeals to you.