The boat conformation of 1,4-dimethylcyclohexane is quite stable because the 2 methyl substituents are far from each other

Answer:

There are actually two conformations to 1,4 - dimethylcyclohexane. One where both methyl groups are in the axial position, and one where both methyl groups are in the equatorial position.

The conformation being asked is the one where both methyl groups are in the equatorial positions because you said the 2 methyl substituents are far from each other. So the answer to your questions is true.

If you flip the boat up, you'll notice that they will now be both in the axial position and will now interact with each other due to the space and eclipsing of bonds, so that conformation will be less stable.

Stability in conformations is where a molecule can be free from steric hindrance, and/or eclipsing of bonds. In boat/chair conformations, you actually want bulky substituents to be in the equatorial position because that's where they are free-er or interact less with the other substituents, hence, more stable. If the substituents are in 2 adjacent carbons, you can use the newmann projection to see if the substituents can find a conformation where they don't interact, and that will be the most stable.