Winning isn’t everything. But it’s something.
Of course, there is no harm to let your little one win every so often, so long as they have no idea that you are. If they catch on to your less than 100% effort or you’re altering the rules to suit her, then you’re showing her that the rules are not as important as winning, and that trying hard isn’t ideal.
Although it will help their self-esteem to feel the taste of victory, she needs to learn about losing, and the best way to do that is to lose… alot.
Instead of just letting them win, acknowledge your advantage, and instead offer a handicap, such as a head start of an extra go if you’re playing a new game. If you agree on the changes, then you’re no longer letting her win. You’re just making the game fair.