Islamically, there really aren't any restrictions on what you can bring into the masjid as long as they're not najis (ritually impure, e.g. covered in filth). For most masajid with restrictions on what is allowed onto the prayer floor, those restrictions are more concerned with keeping things tidy and not disturbing others, and are likely to vary between locations.
There may be concerns if your helmet has images of animate objects on it, but as long as they are covered or concealed (e.g. in a bag) there is unlikely to be an issue.1
Of note, it is also the sunnah of the prophet for people to pray using a sutrah, which is an object that is placed in front of them while they pray such that anybody can walk past that object without interrupting the prayer. A helmet, assuming it's free of questionable attributes, would be perfect for that.
1 Whether having a helmet with such images would be considered sinful is a matter of some dispute, but is tangential to this question as it would apply regardless of if you're in a masjid or not.