They're good as long as they don't feel forced. A time skip between a backstory and the main plot makes sense, or a time skip of several years in which characters have aged.
A time skip has to be really important though. If you can do it without one, don't throw it in just for the sake of it. An example would be, if all the things that happened in the past could instead have happened closer to the current time. If you can just rearrange the plot to not include one, it's better to do that. Otherwise it just feels silly.