Lemon juice is a natural bleaching agent, and it also has some mild exfoliating properties, due to its acidic nature. Therefore, if you have a scar that is thickened or darker than the surrounding skin, lemon juice might have some effects on the thickness of the scar and may lighten its color a bit. However, these effects are probably pretty minimal for most people.
Unfortunately, there are natural remedies like lemon juice that very significantly speed up the fading of a scar. Most of the time, a scar needs to be just left alone to heal and fade on its own, a process that can take several months in most cases.
If you have already waited a long time and are finding that the scar is still quite persistent, then your other option would be to set up a visit with a dermatologist
to see if there are any potential scar revision treatments that would be right for you. For example, dermal micro abrasion and laser based skin treatments are both strategies that some dermatologists use which have a pretty significant impact on the appearance of scars, although both approaches are quite expensive.
Start by seeing a dermatologist at your earliest convenience!