Exactly what the title said. Is this run literally mashing enter the entire time? I know you all are probably biased but is it actually gonna be fun if I invest my time into it

Well, I've only done a few runs, but it's not just mashing the entire time. I think that you shouldn't run a game because you heard that it's good for speedrunning, I think you should speedrun a game if you like the game, and you like speedrunning. It's also fun to see just how broken the game can get, and the community is also really nice. However, as you said I am biased, so only run the game if you want to. Maybe take a look at a speedrun of the game to see what the run has to offer.

Text mashing in some form is going to be a big part of any RPG speedrun but you get desensitised to it after awhile so I wouldn't worry too much about it. If it's Undertale itself that interests you that's the main thing - I havn't run too many categories but I really enjoyed my time here while I was focused on it. If none of the main categories interest you there's also extras over at https://www.speedrun.com/undertalecategoryextensions