I don't think it is a stupid question! In fact I have asked it before. You must all have very well-behaved children if they don't do these things. Whenever I am at a toddler group I see loads of children picking their nose and eating it and other similar yucky things. My daughter is the same. I also know lots of grown ups who do it. Also, sometimes my daughter has a snot explosion (when she sneezes and LOADS comes out) and she / I can't get to a tissue quickly enough so some goes in her mouth.
I don't think these things make her particularly low brow or gross, she is just 3. I do stop her if I see her doing it and tell her it isn't very nice, but I know she does it behind my back. Ditto things like eating insects (yes, she has done that too).
I am sure all these things have protein in but it is such a small amount it never seems to have made a difference to her levels.