The answer is probably. If someone decides to paint your portrait (retrato) you are a sitter (modelo).
You might also have worked as a “babysitter” (canguro).
If something is a “sitter” it is something easy (cosa fácil). Examine these:
– The goal was a sitter (fue un gol que se canta).
– You missed a sitter (erraste un tiro de lo mas fácil). This means that you missed something that was obvious or easy.
So, have you been a sitter? Or has something been a sitter? Probably.