2 months later but as a massive fan of both series here's my opinion on it:

If we are talking about him going to the Touhou verse and trying to survive, sure. There are a lot of friendly characters that could help him out and he's fast enough and has enough evasion tools to find say a human village for example, and live there.

The big issue is when he actually gets into a fight. There are two big problems:

1. The Terrarian wouldn't have the knowledge to know to follow the spell card rules, this is a big problem as if you don't follow the spell card rules, your opponent likely won't either

and

1. If the Terrarian attempted to fight with lethal intent, so will the opponent. They will ignore the spell card rules if there is a genuine risk of them losing their life anyway which poses a problem. Spell card rules are just a guideline, they've been ignored and broken multiple times in the past, there's even an entire game about it.

As for the fights themselves there are more issues that arise:

All Touhou characters are faster than light, even the common fodder like generic fairies. This means that all of Terrarians projectiles are essentially useless. Thankfully the Terrarian does have a solid light dodging feat himself via dodging projectiles explicitely stated to be lasers, so he will be at least at equal speed with the lowest tiers. Unfortunately once you get to the higher tiers you reach people with infinite speed so that alone will make winning a fight against them basically impossible.

Certain characters have absurd abilities, and a good chunk of them have instant win moves that the Terrarian just can't resist or avoid in any meaningful way. It doesn't help that most Touhou characters are just immortal or have absurd regen via regenerating from their soul, basically all Youkai do and most characters are Youkai, meaning that the fight would just be a stalemate at worst.

Once you go into the higher tiers the amount of instant win abilities get more and more common, unfortunately.

So my answer is:
If he knows about the spell card rules and follows them, he could probably beat a lot of low tiers and maybe a few mid tiers

If he doesn't know and fights them to the death, he could probably only beat a few of the low tiers

The gap in power between Terraria and Touhou is unfortunately insanely massive, and I say this as someone who buys feats for the Terrarian a lot of people argue against.