From the support FAQ page of Instapaper (following the normal software-specific support Q’s):

Why does my iced coffee taste so bland?

It’s probably diluted with too much melted ice. Brew it more strongly and chill it before serving — never pour hot coffee over ice.

The ice should only be used to keep it cold, not to make it cold.

You can also try cold-brewing, although mine didn’t work out.

Nice one Marco.