Depending on the length of the deal, this is a good move. If it is a contract for a year or two, that means that Connor Hellebuyck is the long-term goalie and Mason is here as someone who can mentor him and let him have more seasoning. This partnership is also reliant on Mason not starting 60+ games every year. Instead, starting 50-55 and letting Hellebuyck play in the remaining games would allow for Mason to hopefully stay healthy and Hellebuyck to keep developing.
All in all, this is a good move for the Jets. They finally get a starting goalie and Mason gets a starting gig. Hellebuyck gets a mentor who has struggled after finding success, only to re-find his game. Unless the price is high, this is a smart signing.