If you live in a state with taxes, you’ll probably get a higher return with a state/local tax exempt fund like FDLXX, which is at 4.95% APR = 5.06% APY. This can be used like a core for purchases, such that if you pay a credit card or other bill, it will be automatically deducted from FDLXX like cash, without directly selling.
A money market is fine for a simple an easy way to use short-term cash to pay bills or to hold between investments, but if you really want to optimize, I’d favor something that doesn’t have a 0.42% expense ratio, such as purchasing t-bills directly, which also can be done through Fidelity. Without the expense ratio, short-term t-bills have APY of ~5.5% and are also state/local tax exempt.
This is fine for an after tax brokerage, but I wouldn’t recommend it for a Roth. You want something that has a high expected average return in the Roth, to take advantage of capital gains being tax exempt. A money market is the opposite.