Is the microwave scene in Batman Returns realistic?

The delay in the spray cans exploding is believable, given this YouTube video of an exploding axe spray can.

The question then becomes what mixtures of gasses in the spray cans could explode so violently.

This YouTube video of a single exploding aerosol spray can becomes a rather impressive small fireball.

Aerosol, especially oil aerosols, would act as a fuel. That said, a batch of aerosol cans would likely use up all of the oxygen before becoming a huge explosion. That said, there are oxygen spray cans... That seems like a feasible combination to get a larger explosion.

That said, I’ve no evidence on how many cans would be needed to get to this size of explosion.

It is possible that while out of frame she kicked open a gas valve. Fill the room with natural gas, put metal with a flammable gas in it into a microwave, turn on the microwave and it's very believable that the events happened. It's not outside of the realm of possibility.

It's widely known that even tin foil inside of a microwave creates sparks. Metal cans with flammable contents would surely explode inside of a microwave, and that explosion could cause a chain reaction with a room full of natural gas, which would be powerful enough to blow up a building as the combustion followed the pipes throughout the building.

Real explosions in movies are extremely rare, because real explosions do not generate that much visual effects.

Just from the short clip, you can see the entire building engulfed in flames, then everything is pushed out, without a resulting vacuum pulling things back in.

This isn't even a real explosion at all. It's a giant gas fireball.

Things exploding makes noise, and damage is done primarily through kinetic force. Fire is a secondary effect and is usually not effective because the kinetic force displaces air and creates a vacuum afterwards, putting basic fire out.

If you use fuel that can burn in a vacuum, then just go make napalm and forget the explosion.