Looking at sitcoms like The Big Bang Theory, How I Met Your Mother and other ones, up-to what extent do they reflect the culture of America? Is it good to make a statement about other countries' culture after watching these serials?
First of all, a sitcom is about one thing, being funny. In order to be funny, sitcoms sometimes play off of stereotypes about cultures. Stereotypes should never be used by one culture to judge another culture.
An excerpt from the wikipage on the subject:
A stereotype is a thought that may be adopted about specific types of individuals or certain ways of doing things. These thoughts or beliefs may or may not accurately reflect reality.
The only thing you should really get out of a sitcom is a chuckle or two. If you aren't getting that, what is the purpose of watching it in the first place. They are definitely not a place to judge another country or culture.
The complex answer is yes and no. Sitcoms are "situation comedies" and therefore require a comedic value to situations. Let's be clear, the average person doesn't have 23 comedic situations per year. There may be that many or more misunderstandings, but most misunderstandings in romantic relations (let's skip politics) can be corrected before it escalates to the nature of sitcoms. Sitcoms often place a timetable based on someone not "coming clean"/clarifying what happened. Americans generally admit mistakes that they recognize in their actions and wish to correct them.