Validity rollups and Mimblewimble both enable "transaction cut-through", a technique where if A transfers X to B and B transfer X to C in the same block, block producers can collapse that down to "A transfers X to C" and get rid of B, which reduces the amount of data in the block. Other than that I'd say these protocols are quite different from each other and serve different purposes: validity rollup is a protocol for securely transferring assets between two different blockchains; Mimblewimble is a protocol for reducing the amount of information that is stored (and publicly legible) in a blockchain.