Yes, it would be, if requested by the insured (the borrower of the mower). The standard homeowner’s policy (HO-3) covers property of others while it is on the premises that the insured occupies.
The standard homeowners policy is broken down like this:
Coverage A – “Dwelling Replacement Amount.” The amount that the insurance company estimates it will cost to replace your home.
Coverage B – “Other Structures” = 10% of Coverage A (dwelling replacement amount). Any structures that are not physically attached to the house. This includes things like a shed, detached garage, a fence, or a deck.
Coverage C – “Personal Property” = 50% of Coverage A (dwelling replacement amount). This coverage applies worldwide to property the insured owns or uses. Property that is usually located at an insured’s residence other than the primary residence, is limited to 10% of Coverage A (dwelling replacement amount).
Coverage D – “Loss of Use” = 20% of Coverage A (dwelling replacement amount). Loss of use, is the limit that applies to the necessary expenses after being relocated from your home, after a loss. If a fire takes place at your home and you’re forced to live in a motel temporarily, and eat out at restaurants, while the renovation takes place, your loss of use coverage limit will pay out.