No. Sidechains are their own blockchain systems with entirely separate consensus mechanisms. Optimism is verified by a series of smart contracts on the Ethereum mainnet. Because all the necessary data is written to the blockchain, we inherit Ethereum's availability, and as long as there is at least one honest verifier, Ethereum's integrity.
Unfortunately. There is a tradeoff between security and cost, with Ethereum being very expensive and very secure, sidechains being a lot cheaper and usually a lot more centralized and therefore less secure, and rollups sitting in the middle with security that is about as good as Ethereum's and costs that are somewhat higher than those of sidechains. using Ethereum also means we have to pay some Ethereum gas, which causes our transactions to be more expensive than those on a sidechain.