"Sugar is sweet."
Translation:Sukero estas dolĉa.
"Sukeri" is indeed a real word and is registered in PIV. Things like "glacio malvarmas" and "sukero dolĉas" are possible but not all that common. It's important to keep in mind that you can't just tack a verb ending onto a root and have it mean what you want it to mean.
In addition to sukeri (putting sugar in coffee, for example), there is sukerumi (which PIV defines as coating with sugar - even though usage supports it for use while putting sugar in coffee.)
Occasionally you'll also find "sukerigi" for this.
A verb in Esperanto is the action associated with a root. In the case of sugar, it seems like a pretty solid case can be made that the action associated with sugar is to sweeten coffee. In English, who wouldn't understand something like "I wish you'd let me sugar my own coffee"?