According to The Story of Marvel Studios: The Making of the Marvel Cinematic Universe this show (at least the first season) is part of the main MCU timeline. Not really surprising since Marvel Studios and the Russos was involved with the show's first season, and Jarvis made an appearance in Endgame. But it's nice to have the confirmation.

https://thedirect.com/article/marvel-mcu-canon-agent-carter-show