It’s important to do everything well enough to create a book or screenplay worth reading: plot, characterization, dialogue, description. But most of us do a couple of those better than the rest. If you can strengthen your weak points, great. However, it’s more important to capitalize on your strengths.
An example I use frequently is the work of Elmore Leonard. He writes crime novels with vivid characters and dialogue. The plot often feels secondary, but his people are great. That’s why I enjoy reading his books even though crime fiction is not my favorite genre. Leonard is hugely successful.
A contrasting example is Mary Higgins Clark. Not a good stylist and I find her characters thin, but she’s terrific (at least in her early books) at keeping you on the edge of your seat to find out what happens next. Her chapter endings propel you into the next chapter and suddenly it’s four o’clock in the morning.
Choose a genre that displays your particular talent. If you are superb at dialogue, don't write an action/adventure piece. If comedy is your forte don't feel you have to become a master of drama.
What do you do best? That’s what deserves the majority of your time.
(For help using your time most effectively, get my book, "Focus: use the power of targeted thinking to get more done." You can find it at Amazon and other online and offline book sellers.)