[RPG] What proficiencies do you need to craft Serpent Scale Armor

For crafting non-magical items.

From the Downtime Activities on page 187 of the PHB:

Crafting

You must be proficient with tools related to the object you are trying to create (typically artisan’s tools). [...] For example, someone proficient with smith’s tools needs a forge in order to craft a sword or suit of armor.

So RAW it would appear that crafting doesn't actually require any ability checks. You just need proficiency in artisan's tools to be able to craft non-magical items in the first place.

But the DM might call for other ability checks that require artisan's tools.

From page 154 of the PHB:

A tool helps you to do something you couldn't otherwise do, such as craft or repair an item [...] For example, the DM might ask you to make a Dexterity check to carve a fine detail with your woodcarver’s tools, or a Strength check to make something out of particularly hard wood.

There's no real contradiction here. In general, anyone can use a tool with or without proficiency - if they have proficiency, they get to add their proficiency bonus to checks made with that tool. For example, from artisan's tools:

Proficiency with a set of artisan’s tools lets you add your proficiency bonus to any ability checks you make using the tools in your craft.

However, as you've already discovered, crafting has specific rules:

You must be proficient with tools related to the object you are trying to create (typically artisan’s tools).

So in general, proficiency with tools isn't required to use those tools, and Mearls' answer reflects that, but in the specific case of crafting, proficiency is required.