As a reminder of what Wall of Fire does:
You raise a blazing wall that burns creatures passing through it. You create either a 5-foot-thick wall of flame in a straight line up to 60 feet long and 10 feet high, or a 5-foot-thick, 10-foot-radius ring of flame with the same height. The wall stands vertically in either form; if you wish, the wall can be of a shorter length or height. Everything on each side of the wall is concealed from creatures on the opposite side. Any creature that crosses the wall or is occupying the wall's area at the start of its turn takes 4d6 fire damage.
One important thing to note is that the wall is only 10ft tall, so it seems easy for a huge creature to attack from above it. In Pathfinder 1, it was indicated that a Huge creature was typically 16 to 32ft. I haven't found such a specification for Pathfinder 2 but there is no reason to think that changed.
As a DM, I would consider that a 10ft tall Wall of Fire could be considered tall enough to constitute a lesser cover, but nothing more. As I understand the spell's description, the concealment relies on seeing through the fire being difficult and thus should not apply to a creature that doesn't have to look through it.
Still, I think in the specific case where the Huge creature can't attack above the wall (let's say for example the roof is low on this specific point) things get more interesting. The rules don't really cover this: reach doesn't explicitly trigger the attacker "crossing the wall" although it could be interpreted as such. I personally would rule that as triggering the effect and dealing 4d6 fire damage to the attacker, but wouldn't be surprised if other DMs ruled it otherwise.