While by no means that anything is in the works for the series, this does come as good news after the rather sudden announcement of bankruptcy for Alphadream back in October. At the very least, it could mean that Nintendo may still have plans for the series. At worst, they might just want to protect the series name for now.
While rather standard for video game trademarks nowadays, it should be noted that the trademark also covers mobile game purposes, meaning that this could be a possible option.
The last released game for the Mario & Luigi series was Mario & Luigi: Bowser’s Inside Story + Bowser Jr.’s Journey on Nintendo 3DS.