What you need to look into here are the various material properties and the reasons you might want them in armour.
Strength: the strength of material is the amount of force it can withstand and still recover its original shape;
Hardness: the hardness of a material defines the relative resistance that its surface imposes against the penetration of a harder body;
Toughness: toughness is the amount of energy that a material can absorb before fracture.
Stiffness: the rigidity of an object — the extent to which it resists deformation in response to an applied force.
Now these properties are all very well, but they're not interesting within limits. They only become interesting at the point of failure.
Ceramics score fairly well on hardness, toughness, stiffness but very badly on toughness. Once the limit is exceeded it shatters and absorbs no more energy.
Steel doesn't score so well on hardness and stiffness, but it scores a lot better on toughness. When its stiffness is exceeded it deforms, first elastically, then plastically, absorbing ever more energy as it does so before ultimate failure.
We don't tend to wear steel as personal armour in the modern age though so let's compare ceramic armour to aramid fibre better known as the variety and brand name Kevlar.
Aramid is a very soft cloth with a very high tensile strength, it's immensely tough. The reason it protects you from bullets is that on impact the threads don't break, ensuring that the projectile remains outside your squishy parts. It doesn't protect you from the force of impact though, you'll get quite a bruise, and it's vulnerable to sharp edges, it won't protect you from a knife or a bullet with a sharp point.
Ceramic armour is much harder and stiffer, but with its relatively low toughness it protects you sacrificially. The energy of the impact is absorbed by the shattering of the plate. It protects you from armour piercing rounds, once. After which you need to replace the plate.
So which do you choose for your personal armour?
Both. Ablative ceramic plate over aramid cloth. A single material gives only a single set of properties, if you wear both you get both.
You're asking to be able to survive tank shells though. Tank shells are designed to kill tanks. You need to be carrying enough protection to survive that. Nzaman has given you basic values for mass and momentum, that's your first consideration.
A tank will weigh around 60,000kg, perhaps you won't want to be trying to carry that much weight on a mecha as you'd need to spread the weight, legs will just sink in, but it gives you an idea of the values you're competing with.
However ceramics come into their own in tank armour, their high hardness helps them resist shaped charges, ceramics alone won't protect you, but they are a critical part of your armour sandwich.
To armour your mecha you're going to want multiple layers of yes, ceramics, but also composites and steel giving you an equivalent of Chobham armour the different properties of the different materials in this sandwich will give you the best chance of surviving the attack.