In general, mislabeled albums are not as valuable as albums that have the correct label. This is because mislabeled albums are often considered to be defects, and they may not be as sought after by collectors. However, there are some exceptions to this rule. For example, mislabeled albums that are particularly rare or that are associated with a famous record label may actually be more valuable than correctly labeled albums.
Ultimately, the value of a Beatles Sgt Pepper album with the wrong label on it would need to be determined by an expert in music memorabilia.