Do you mean in general, or as a one-off encounter? I'm sure I've come off as being incredibly dull at times -- if I'm thinking hard about something in my work, I can get very preoccupied and come across as a bit vacant. To put it mildly. 
If you mean that prolonged contact with the person in a variety of moods would still reveal them as dull, I suppose in my case I find people who lack awareness and self-awareness dull -- an observant person who can talk well about their life is never dull.
We've been doing renovations to our house for the best part of a year, and I have absolutely nothing in common with our contractor, whose life consists of working six long days a week on sites and being asleep, so in theory, pretty dull -- but he's brilliant fun to talk to. He's a great storyteller, very observant, an acute judge of character, funny.
My brother's girlfriend, on the other hand, whom I try so hard to like, is desperately dull, pretty much exactly what @MsTSwift has described. She talks in clichés. She repeats stories that really don't even bear telling once, and if you don't express the requisite surprise and impressedness at some minor coincidence, like her seeing a former teacher of her's on the street in another city, she'll keep repeating it until you do. Aargh.