how simple can you make a monster?
the simplest possible system that still cares about combat numbers might have every monster just be health, damage, effect. crunchier rulesets might necessitate movement, level, dodge/armor, skills, languages, attributes (as in STR/DEX/INT/etc), morale, alignment, whatever.
look at each of the items listed above. all but one of them are bones; one single item there is meat. try and guess which one is meat before reading ahead.
bones are everything in a monster statblock that must be there because it's part of the system template. you can cut down on bones by making some mechanics (e.g. attributes, skills) player-only, or by having some things only listed if they deviate from the default (e.g. only list movement if an enemy's movement is extra fast or slow; otherwise it's standard).
you need some for a system to function (unless your system is fully descriptions and no numbers, i guess) but cutting out unnecessary bones reduces clutter. the less stuff there is to keep track of, the faster the GM can find the specific stuff they're looking for. more bones can be worth having - crunch isn't always bad - but it must justify its existence.
meat, on the other hand, is the stuff unique to an individual monster. i listed the simplest system for a monster as "health, damage, effect"; and "effect" is the most important thing. that's the meat. that's the part that actually matters.
differently-shaped bones are not meat. one monster might have high damage and low health, and another might have high health but low movement, and these differences matter and can be interesting - but they're still just bones. they can't take your enemy variety very far.
what does constitute meat? any unique trait that can't be achieved by just tuning numbers in the template up or down.
maybe a monster can see and hear through walls. maybe it knocks you back one movement every time it hits you. maybe it can turn you into a frog. maybe it can stick to walls. maybe it's just immune to damage and you have to find ways to neutralize it that aren't damage.
this advice applies both to crunchy tactics games (where you solve problems with the rules) and looser OSR games (where you solve problems with clever in-fiction solutions). what makes the best meat differs from system to system, but the goal is the same if you're in any game where "fighting monsters" is a thing the game cares about.
