The DM can create any equipment they want, magic or not.
The Goblin Helm as described is not an item in any of the core books, but that doesn't mean that it doesn't do exactly what your DM told you it does.
Generally, a helmet is considered part of the armour (eg, Plate Armour comes with a Plate Helm, which is factored into the AC), but again, the DM is always able to house-rule or create new items for the players to use, whether as a magical item that gives a +1 AC bonus as an enchantment, or a mundane item that is just sturdier and more protective than average equipment.
The only concerns a DM must take when creating new items, rules, spells, etc.. is game balance, and party balance. In this case, if it is non-magical, it is strictly better than an equivalent magical item with a +1 AC bonus, as it will never use Attunement, it will always function in an anti-magic field, and it can still be enchanted, on top of its mundane bonus. It would not benefit from the resistance to all damage types magical items enjoy however.
Additionally, giving out +1 AC early in the game can be considered a bit strong of a reward due to bounded accuracy, however this is not inherently a problem. (Some campaigns run high power with tons of magic items and rewards, see Monty Haul [TvTropes Warning!])
As long as encounters are balanced around this fact, everyone at the table is happy and having fun, and no one in the party feels underpowered as a result, you can continue to enjoy your sturdy goblin helm.
Related: Rule Zero, Will adding helmets unbalance the game?