A riding lawn mower can be tipped on it's side to work on it but make sure you tip the riding lawn mower on it's side where the carburetor is not.
If you tip the riding lawn mower on the side where the carburetor is then the oil could leave the engine and go into the carburetor.
So tip it so that the carburetor is on the side up in the air.
For a push lawn mower don't tip it on it's side but instead raise it up so that the spark plug is up in the air.
That will keep oil out of the carburetor.
If possible it's also best to just use a lawn mower lift or something to lift the riding lawn mower up in the air with the front of the riding lawn mower raised up in the air.
Tipping the riding lawn mower on it's side is not recommended and it's best to raise it up in the air so the front is up in the air to work on it.