Is it worth it to replace the stock blade of a miter saw?
While I can't speak to the quality of all manufacturers blades, in general the blade that comes with the saw is of no lower quality than any other blade offered by the manufacturer.
Most miter saws come with a 24 tooth FTG (Flat Top Grind) blade, which is good for fast course cuts. If you're looking to chop 2x lumber, this blade will work fine for you. If you're planning to use the saw to make fine cuts, you'll want to upgrade to a blade with more teeth and a grind that reduces tear out.

More About Circular Saw Blades.
Number of teeth
In general, the more teeth a blade has, the finer the cut it will produce.  A blade with 24 teeth; for example, will remove more material with each blade than say a 40 tooth blade. because more material is removed more quickly, the quality of the cut is lower.
Grind
The grind of the blade, describes the shape of the tips of the teeth.
Flat Top (FT)
As the name describes, the tips of these teeth are flat, or straight across. These blades produce the courses cut, and often result in lots of chip out.

Alternate Top Bevel (ATB)
The teeth of these blades alternate between a right, and left handed bevels. If you look at the teeth, you'll see that one is beveled one way, while the next is beveled the opposite way. These blades produce a finer cut that FT blades, and result in less chip out.

Combination (COMB)
These blades consist of groups of teeth, where one will be FT with the rest being ATB. Typically there will be a group or 5 teeth (1 FT, 4 ATB), where each group is separated gap (gullet). These are more general use blades, and do fairly well at both ripping (cutting along the grain) and crosscutting (cutting across the grain).

Triple Chip Grind (TCG)
The teeth of these blades alternate between a FT tooth, and a double beveled tooth (bevel on left and right, with a flat top).  They are good for cutting hard materials, so are often used to cut MDF, plastics, and laminates.

High Alternate Top Bevel (HiATB)
These blades have alternating beveled teeth like an ATB blade, except that the bevel angle is much greater. An ATB tooth is typically beveled at 10-20°, a HiATB tooth will be beveled at 25-40°. These blades are good for extra fine crosscutting, and reduce chip out to a minimum.

freud blades used for example purposes only, because they show the tooth configuration on the blade. I have no affinity towards freud.