I was looking at this wiki post, and it said that Skeletron masks are the only masks that can be obtained outside of PC. However, I was wondering if that meant that it is obtainable on mobile. It never says exactly what devices it is available on, so it might be possible, might not be. I have probably killed around 10 Skeletrons so far, but have had no luck receiving a mask.
Terraria – Are Skeletron masks obtainable on mobile
terraria-mobile
Related Solutions
The game has a total of 21 NPCs that can move into your house. They are:
- The Guide
- The Merchant
- The Nurse
- The Painter
- The Dye Trader
- The Demolitionist
- The Dryad
- The Arms Dealer
- The Party Girl
- The Goblin Tinkerer
- The Witch Doctor
- The Clothier
- The Mechanic
- The Angler
- The Traveling Merchant
Additionally, in hardmode you gain:
- The Pirate
- The Truffle
- The Wizard
- The Steampunker
- The Cyborg
- The Tax Collector
Finally, there is one event NPC that do not live in housing:
- Santa Claus (shows up randomly between Dec 15-31st)
If you look at the wiki you can see a list of all the NPCs in the game. You can tell which NPC is only available in the PC version because these NPCs have a small icon of a computer directly after their description. It looks like this: 
According to the Terraria wiki, all maps in Terraria mobile are the same size as small worlds on PC. This is 4200 blocks wide by 1200 blocks tall.
Best Answer
Yes, Skeletron masks are obtainable on mobile. Maybe you just haven't had the best of luck so far, because it should be a 1/7 chance, but eventually you will find a mask that looks like this: