My view? It's worth going the extra mile for guests - unfortunately, that sometimes comes with more work (which we could all do without). Guests appreciate extra touches to make them feel just a bit more special, so those extra big, fluffy towels for the hot tub could prevent complaints and even push up your reviews (which is never a bad thing)!
If you don't want to purchase/launder more towels, then make any future guests are aware via any communication you have prior to their booking/arrival. Keep it light though, something along the lines of "Hey, we have a great hot tub for our guests to use, you'll all love it - just make sure you bring along a bath towel each and you can use it whenever you like." Be informative - and nice!
I found this blog handy if you're after a few more tips on making your guests feel at home. : https://www.airbnbtips.com/boost-airbnb-ratings/
Good luck - P.S. a hot tub is a real bonus, so I would make a real feature of it!