What's the name for when a layman thinks a technical task is easier than it actually is?

I believe what you are looking for is called the Dunning–Kruger effect. I would quote the Wikipedia page but I think Theodoros Chatzigiannakis puts it better:

the Dunning–Kruger effect (...) is a cognitive bias that seems to apply to any activity, ranging from (e.g.) understanding a piece of text to being a doctor. The bias is that people who are unskilled at or inexperienced in any particular activity will overestimate their own skill level and underestimate others' skill level. In other words, being unskilled at or inexperienced in something not only prevents one from performing it, but it even prevents one from accurately evaluating anyone performing it.

From: graphicdesign.stackexchange.com (Why is it that people are so surprised that graphics design can be hard?)

The term "armchair quarterback" is defined by oxforddictionaries.com as

A person who advises or offers an opinion on something (especially a sporting event) in which he or she is not actively involved, or about which he or she lacks first-hand or specialist knowledge.

The term is related to the sport of American football, in which the player in the role of quarterback makes the most significant tactical decisions in any given offensive play. Keen amateur spectators of the sport, despite lacking any first-hand experience playing the position, especially at the professional level, often harshly criticize the decisions that the quarterback has made, the implication being that the spectator would have made better ones if he or she were in the quarterback's place.