The writing includes the entirety of the experience, not just the ending.
It's not like the SMT negotiation system at all. I mean, at a basic level, your goal is to get your enemy to go away, but negotiation in SMT is fundamentally different in that it exists to supplement an existing fighting system, and is semi-randomized to keep battles from being 'figured out' entirely. Undertale's battle system is an extension of its' focus on puzzle and problem solving. Enemies exist to be solved, and your most basic input (using the FIGHT command) is the most straightforward and least dynamic way of solving them. It's figuring out the 'character' of an enemy and taking the appropriate choices in the right order. It's consistent, and every 'conversation' follows a logical chain of events, unlike SMT's negotiation.
It's anything but 'dumbed down' from SMT's, but I don't know how you'd qualify that because IMO SMT's negotiation mechanics are already very 'dumb' in the first place.
When you 'figure an enemy out' in Undertale, the solution never changes. Which can be boring after a certain point, but the random encounter rate is so low and there's enough variety in enemy types that it never really becomes too big a problem. It helps that the basic mechanic of combat - guiding your character around a box to avoid bullet hell patterns, is something I found really fun and creative for an RPG, and the game is never afraid to mix up its' own mechanics.
Just try to go in without any expectations set for yourself. Not what you read in a tweet, not some hyperbolic review saying best game of forever. Just play it and reach a conclusion in as big of a vacuum as you can.