Do the monuments from Amonkhet reduce bestow costs?
No. By the time that you would apply the cost reduction from the Monument, the bestow card is no longer a creature spell.
To determine the cost of casting a given spell, we have to have a few pieces of information. First of all, is it being cast for its mana cost or an alternate cost such as Bestow? If there’s X in that cost, what’s the value of X? Are there any additional optional costs that need to be paid, such as Escalate or Kicker? Once we have this information, we take the mana cost or alternate cost of the spell, add the optional costs, add any cost increasing effects (e.g. Thalia, Guardian of Thraben), subtract any cost reductions (e.g. Monuments), and apply Trinisphere if it’s around and the cost would be less than 3. (Yes, it gets its own unique step here.)
In the case of a spell with Bestow, as soon as you declare your intent to pay that Bestow cost, the spell stops being a creature spell on the stack and becomes an aura spell instead. You then choose a target for it and then pay its costs. Since it’s not a creature spell at this time, the effect of any of the Monuments won’t apply anymore. Effects that reduce the cost of Aura spells will apply, though.