For celebrity - When celebrities enter politics, it leads to a great disorder. The stars are not used to the publicity they will be getting as a politician. They might not know how to run a country or a state smoothly, which will lead to a rough end.
For normal person - People usually vote for their favourite actor and actress according to their looks, acting and dancing skills and popularity level. But people forget that this is politics, not a star meter. Here you have to look at the advantages and disadvantages of a celebrity. The type of a person, he or she is, is also a huge matter to think about.
Mainly, the point is that a normal person is influenced by the person's reel life personality, not the real life one.