I do think that having the popular posts gadget in your sidebar is a good idea. I have put it just under the search box. It gives people visiting your site an easy snapshot of what your best ie most popular posts are about.
You add this to your blog, in Blogger, by going to your Dashboard, clicking Layout and then clicking one of the boxes labelled add a gadget. Blogger then comes up with some to choose from. If you scroll down you will find it is 5th on the list. Just click the plus sign and make a couple of selections as to what you want shown in terms of time and number - I suggest thumbnails and titles and 10 posts (the maximum). Then you just click the orange Save button and you are done.
You can drag the gadget around the Layout screen until you have it where you want it.