Look, maybe. But the spell itself doesn't mention a lake of lava, and nor does Jeremy in his tweet response. And no rule dictates that this magical illusion lava must do 18d10 damage. This is all within the realms of DM ruling. The spell says "The terrain's general shape remains the same", so can a lake be summoned? Is lava even a "terrain", or is it a hazard that might form an acceptable subsection of some terrain with a volcano.
Personally I might allow this as part of a one-off well planned trap, but not guarantee it works every time. Generally I would probably rule that the terrain changes wrought by Mirage Arcane could create dangerous, even damaging terrain, but would not appear and suddenly cause damage to any creature. Like, an enemy walking over a field could suddenly find themselves trapped on an island surrounded by lava, but not actually drowning in the lava itself. This is my ruling only, but the books say nothing definitive either way.
Yeah I do find it odd how Jeremy doesn't directly respond to the lava part, but his response implies yes, it does cause damage, as he only gives other positive examples. I guess it's fair that there's nothing saying that the damage would be equal to real lava, but I got the impression that these things are actually happening. Like you can die from drowning in the lake, not from psychic damage from you thinking that you are drowning. Obviously the 18d10 is a suggestion for lava from the DMG, not a rule necessarily.
In my opinion, this is something that'd be strong against weak, but weak against strong. While it could decimate armies of ordinary soldiers, the spell can also be removed with a single Dispel Magic from a half-decent magic user. It'd also be pretty ineffective against enemies with flight, and creatures with true sight can bypass it altogether. If I was running a war campaign, I'd say that any sizable force would have some magic users, so if the player wanted to use this they'd have to account for enemies that could disrupt it.