The ruling on damage depends on how you judge some of the details
From the spell description (Basic Rules p. 90)
Each creature in a 20-foot-radius sphere centered on that point must
make a Dexterity saving throw. A target takes 8d6 fire damage on a
failed save, or half as much damage on a successful one. The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried.
Some ways to peel this onion.
Advantage?
As the DM, you are always in a position to judge whether or not advantage accrues in a situation. If you want to assign advantage on the saving throw, that might be the simplest approach.
Advantage and Disadvantage (Basic Rules p. 4)
Sometimes an ability check, attack roll, or saving throw is modified by special situations called advantage and disadvantage. Advantage reflects the positive circumstances surrounding a d20 roll, while disadvantage reflects the opposite.
A saving throw can be modified by a situational bonus or penalty and can be affected by advantage and disadvantage, as determined by the DM. (Basic Rules, p. 62)
The DM can also decide that circumstances influence a roll in one
direction or the other and grant advantage or impose disadvantage as a
result (Basic Rules p. 57)
A player can argue that a Dexterity saving throw advantage is negated if the Mummy is (ruled by you) to be Restrained in the sarcophagus (can't move, since it's stuck in there). You don't have to rule that way, but that can account for advantage and disadvantage cancelling each other out.
Restrained (Appendix A, Conditions) The creature has disadvantage on
Dexterity saving throws.
You assign advantage based on circumstances.
Making an Attack
Whether you’re striking with a
melee weapon, firing a weapon at range, or making an attack roll as
part of a spell, an attack has a simple structure. {snip}
2. Determine > modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. (p. 72 Basic Rules) (I'll address cover at the end).
Other considerations / versimilitude
Is the sarcophagus flammable? If so, it ignites. If not, then it
doesn't. If it ignites, damage from the fireball makes sense.
If the sarcophagus is sealed, is there a corner to spread around?
Likely no, but if it is not sealed, or decrepit, then the Mummy may be exposed to fire flowing into the Sarcophagus. (Around corners, as it were).
- Do you want to assign HP to the sarcophagus and see if the fireball
destroys it? That isn't necessary, as you could just rule
flammable/not flammable, but it is a choice if you want to gin up
some HP for it based on the DMG section (p. 246-247) that provides HP for things like barrels and carts.
Does the Mummy avoid damage altogether?
You could rule that the sarcophagus provides enough cover so that the fireball can't get to it ... no corners to go around. That's a fair ruling if you rule that the sarcophagus is not flammable, or it is not destroyed, or it is sealed. The Mummy in that case is never in contact with the fire, which would be because the fire didn't spread around the corners to get at the Mummy.
Some problem with the Cover rule in this situation
Do you rule that it has total cover?
A target with total cover can’t be targeted directly by an attack or a
spell, although some spells can reach such a target by including it in
an area of effect. A target has total cover if it is completely
concealed by an obstacle.
The rules for cover are a bit sticky here. While you can say that the sarcophagus completely conceals the mummy from all directions -- and thus does not provide any corners for the fire to go around -- that may not harness the intent of the rule on cover as it is written. It states that some spells (AoE) negate cover. Cover works well against Line of Sight spells, where full cover behind a stone wall blocks the spell, but the AoE provision lets a fireball going off above the wall to spread to the target. Ruling that the sarcophagus is a wall with coverage from all angles fits well enough, but you might not prefer that as that's a soft area in the rules text. You can make a ruling, using the term "cover" in a less "rules mechanics" sense and in a more "normal language" sense.
Make a ruling and play on, based on what makes the most sense to you.
