If my mom can claim me as a dependent but she isn't, do I answer YES or NO when asked if someone else can claim me as a dependent?

If your parents meet the requirements to claim you, let them! If your parents meet the requirements, but choose not to claim you, then you still have to select the option for “I can be claimed on someone else’s return” when you file your tax return.

If you can be claimed, you must answer yes that you can be claimed, even if you don't want to be claimed.  

For 2020, if you answered "no", you would be eligible for a stimulus rebate of $1800, but that would be tax fraud, and if caught, you would be made to repay it with interest and a penalty.  Answering "no" may also change the calculation of the extra tax owed by dependents on unearned income (the "kiddie tax")--this tax applies higher rates to investment income in a child's name so the parents can't save money but putting investments in a child's name to get a lower tax rate.  This would also be tax fraud.

There is a second question of "will the person who could claim you actually claim you this year."  This only affects your eligibility for the American Opportunity Credit, and is really only helpful if the college student dependent has a significant amount of income earned from working and the parents' income is too high to qualify for the AOTC.