Might want to rethink those tires on concrete pads.. maybe prop the mower up, off the wheels. As long as you can keep the rain out, I would think you'd be OK.
"Man is the only animal that laughs and weeps; for he is the only animal that is struck with the difference between what things are, and what they ought to be."
1986 CMX250 <SOLD! to a member 1999 VT750CDX.. The speedo: 1973 VW Beetle
If I had to choose between my bike and my mower, the bike would get the garage. Indoor storage is better for either. It doesn't sound like your mower gets lots of hot/cold temperature cycles, and if you use it regularly during mowing season it shouldn't be sitting long enough to rot the tires most of the year. It would be good to protect it from sun and rain. A small shelter would be much better than a tarp. Even a plywood crate, so it gets air all around it, and nothing resting on it. Set it on blocks during the winter with the tires off the ground. Pickle the engine like you would any other. If meeces are a concern where you live, you might want to consider taking steps to anticipate an Invasion Del Rodentia.