The perception of guilt or pleasure in any activity is highly subjective and varies from person to person. What one person views as a guilty pleasure may be completely acceptable and non-controversial to another, and vice versa.
The mind's eye typically refers to the ability to visualize things through memory and imagination. However, in the context of guilty pleasures, "it's all in the mind's eye" suggest that the actual enjoyment comes from the cognitive rather than physical or observable components of the experience. For instance, one's guilty pleasure can be a hobby, fantasy, interest, or guilty secret that doesn't necessarily manifest in actions but brings mental, emotional, or intellectual satisfaction.
To some people, the satisfaction might outweigh or overshadow the potential feelings of guilty, leading to the expression "it's all in the mind eye"