Eh, lots of variables and generalizations in play... but in a nutshell; if the amp wattage is higher than what the power/watt handling of a speaker is, it's going to lead to 1 of 2 likely outcomes - 1) the speaker will easily distort... which in some scenarios could be a desirable effect and compliment the amp distortion (especially when paired with another higher power/sensitivity speaker); in other scenarios, it could just sound muddy, "farty", or bad. 2) The speaker could just fail...blow the cone out or toast the voice coil.
In the reverse role; if the speaker watt/power handling is way higher than the amp output... usually this will make very little difference. It depends more on the design and sensitivity of the speaker at this point. For example, I love to pair the 300 watt EVM-12L with all sorts of amps, including little 5 watters; fairly transparent speaker pairing that let's more of the amps own voicing be heard (minimal speaker distortion/coloration).
Usually the only negative scenario is where it's a speaker with a very heavy cone and low sensitivity; that could be a speaker that needs a whole lot of power to make it move in order to sound good. There's not many guitar oriented speakers out there that fall into this grouping though. It's more in the extreme cork sniffers high end of car or home audio (or cheap garbage speakers from temu or something).
TLDR - Low power amp with high power speaker; okay. High power amp with low power speaker; may cause death of speaker, or could get you that cinnamon girl sound you've been searching for all your life (if one was to ever search for such a sound).