Improv is not necessary to act well or become a Sitcom actor. A lot of the messaging in Hollywood is that it is necessary. Agents, Managers and Casting Directors will tell you to have it on your resume. I advise people to find a balance. Commercial auditions ask actors to improvise frequently. Theatrical auditions, not so much.
I’m an improviser myself. My first teacher was Lisa Kudrow and I had the opportunity to work with many of the original Groundlings. I went all the way through The Groundlings and the training I got helped me immensely.
It’s worth noting that many actors working in TV and film comedy do not have an improv or sketch comedy background. Either way, it comes down to auditioning well.
People can get over trained in improv. Sometimes they spend a lot of time in those venues without learning how to audition. It is important that actors get familiar with working on camera and the audition configuration.
Remember to think about what your goal is as you take improv: do you want to be on a team? Do you want to join a theatre company? If so, adjust your training and budget accordingly.
Remember to balance improv with on-camera training!