This creature riding middle size??

A medium creature cannot ride a mule. A small or smaller creature can, but the 'beast of burden' feature only helps carrying capacity.

A medium creature is not EXPLICITLY allowed to ride a mule. A riding beast is CARRYING it's rider and geometrically a mule is capable of carrying a human shaped thing, so the rules as written are pretty ambiguous in that regard. For example, it's a fact that in the rules as written, a mule could carry a dummy in the size and shape of a normal human, as long as you secure it properly, which wouldn't be hard. In that sense, making the case that they can't then carry a human seems as contradictory to the rules as the idea that Carrying Capacity applies only to carrying objects, not people. 
And so both interpretations can be considered to contradict the overall rules in some sense, and it's up to the DM to judge.

Mules have been ridden by humans for centuries, medium creatures can definitely ride mules if your DM has any sense.

Quote from vallhundfadewalker >>

Mules have been ridden by humans for centuries, medium creatures can definitely ride mules if your DM has any sense.

 What Walker said.

If you’re trying to ride it at a full gallop for an hour (like the travel rules say you can with a horse), a strict GM might object.

Otherwise, it’s hard to think of circumstances where you’d have that big a problem. 

Mules can easily be the size of horses irl, and carry people for long distances all the time. They've been notably healthier and stronger than horses, but the stats in 5e don't reflect that.

Jesus road a donkey, bro. lol

Quote from Kerishai >>

Mules can easily be the size of horses irl, and carry people for long distances all the time. They've been notably healthier and stronger than horses, but the stats in 5e don't reflect that.

The stats in 5e are really for a donkey, which mules are mechanically folded into, apparently to save space.

Mules are superior to both horses and donkeys for the most part, which is why they are bred even though they are sterile. Disappointing they thought to include donkeys and mules as the same stat block.  If I'm DM, mules are slightly more expensive and have slightly better stats than horses.  I would only use this stat block for donkeys.

Quote from jester1985 >>

Mules are superior to both horses and donkeys for the most part, which is why they are bred even though they are sterile. Disappointing they thought to include donkeys and mules as the same stat block.  If I'm DM, mules are slightly more expensive and have slightly better stats than horses.  I would only use this stat block for donkeys.

 That’s fair.

Could these stats fit a llama too you think?

Yes they could

I have a donkey and I use this stat block.

I think attaching this to a cart and sitting on it would be better?

Quote from Dartagnan_Galochio >>

Could these stats fit a llama too you think?

Quote from DidLV >>

Yes they could

🔜"Why are those llamas wearing hats?"

Quote from Skrux >>

I think attaching this to a cart and sitting on it would be better?

That is likely this creature's intended role.

Each mule can pull 2,100 lbs of weight including the weight of the vehicle.

PHB Chapter 7: Using Ability Scores -> Using Each Ability -> Strength -> Carrying Capacity
Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry.

PHB Chapter 5: Equipment -> Mounts and Vehicles
An animal pulling a carriage, cart, chariot, sled, or wagon can move weight up to five times its base carrying capacity, including the weight of the vehicle.
If multiple animals pull the same vehicle, they can add their carrying capacity together.

Putting these two together
The formula is STR x 15 x 2 (Beast of Burden feature) x5 or
14x15x2x5 = 2,100 lbs

BEST MOUNT EVER!!! For only 8 GP!!! 40 ft move/ 80ft dash is an auto pick-up if I'm playing a gnome, kobold, goblin, halfling, custom lineage(small size), etc. and they are medium so no squeezing through 5ft wide hallways like camels or horses, amazing!!!

Mules can Cary exactly 420 pounds (14x15x2) because of beast of burden.

Llamas son mas grande que las ranas...