Sorry for the machinegunning of questions, but can Anomaly take the form of smaller people (shorter and/or lighter)? If not that would justify it not taking on the angler's form.
They can take on the form of the shorter townsfolk, the reason they don't do it for the Angler largely comes down to mechanics and NPC usefulness.
The Angler doesn't act as a vendor, just gives out quests, whereas any of the other townsfolk who act as vendors at least give the player an option to sell excess stuff while exploring, so even vendors with relatively useless inventories or ineffective attacks would have that as a crucial traveling function.
The decision of who to bring along largely comes down to the efficacy of their self-defense, the items they sell, or personal preference. Naturally, certain vendors would be more 'popular' than others. Having constant access to explosives or ammo or mana potions would be pretty nice to people who utilize those resources, but at the same time the the Dryad’s Blessing aura could be more valuable than anything she could sell. Or maybe someone just really likes the Witch Doctor, idk.
It’s quite a bit to keep in mind... there’s also the possibility of other passive benefits to each one that may be more than what the NPC would have by default (to make using the service a much more tempting offer). The Angler would still be considered ‘useless’ by comparison, which continues to serve the purpose of upholding the game’s decision to not murder children.
The only other townsperson that Anomaly would possibly not mimic would be the Guide, depending on how useful it would be to have crafting schematics on-call, versus plot-armor issues; his otherworldly vast knowledge may not be something Anomaly can mimic flawlessly, but is the crux of the Guide’s entire function.
Tax Collector would be another curious one, since while there could be some benefit to being able to collect the money at any time without having to go home (especially since the amount caps, which loses the collection opportunity after that), actually having gold on you while adventuring is just asking to lose it if you happen to die. Plus, there’s the slightly more meta question of how they're actually collecting that money if they're shadowing the player anywhere other than town (of course, that could just be handwaved by "magic needs no explanation").