The phrase "dullest thing" refers to something that is very boring or uninteresting. It means that it lacks excitement or anything that would grab one's attention. Full definition
But I know lots of ex-player are talking dull things now (Cr 2 OG), and I know that the only three simultaneous award winner, World Player of the year, European Player of the year and African Player of the year, George Oppon Weah, openly acknowledge his input in his worldwide success. (justarsenal.com)
They liked only dull things such as toasters (4), sponges (127), and refrigerator magnets (226)-- and not those cute bunny shapes and such, but informative freebies from the plumber, the electrician and many from the firm where they worked, Beige & Beige. (bookbrowse.com)
For example, those who watched the funny clip said rather revealing things like, «In January I broke my collarbone falling off a pole while pole dancing,» while those who watched the other clips said comparatively dull things like, «I am at Worcester College in my first year.» (mindbodygreen.com)