There is this restaurant on a volcano on Lanzarote, in the "Parque Nacional de Timanfaya", where they cook with the heat from the magma below. I am sure you could cook a pizza with that, as well.
There are places on the planet Mercury that, if you were to find a lava tube of sufficient depth, would be the perfect temperature for human habitation. Some of the craters on Mercury's poles are never exposed to sunlight and actually have ice in them. Most of the planet is of course boiling hot when the Sun is overhead. But there should be some choice areas where you could skirt the balance of the two, and find lava tubes that, with proper sealing, would be quite comfortable for humans to occupy.
A pizza oven is about 450 C, so I'll figure it out based on this graph of the temperature in the Earth by depth.
0 to 500 C = 80 px
6.25 C per pixel
450/6.25 = 72 px
0 to 100 km = 54 px
1.85 km per pixel
Line y coordinate at 72 px x coordinate = 9 px
9*1.85 = ~17 km
The deepest hole we've ever actually drilled is the Kola superdeep borehole, which is a bit over 12 km deep. This is a fair bit short of our ideal pizza oven temperature, but it did see temperatures of 180 C, which is certainly enough to cook a pizza.
The geothermal gradient is different at different parts of the earth. You can probably bake a pizza at much shallower depths at the mid ocean ridge, near a volcano, or even at an active orogeny.
Probably, but is that position also accessible to you to put the pizza there in the first place, and be able to get it back out? Because if it isn't, all you've done is sacrifice a perfectly good pizza. ¯\_(ツ)_/¯
I think the process of transporting it there and back fucks with the temperature because the it increases and later decreases so slowly that it's overcooked just by bringing it there and back again.