Hi abbygrant,
If your mattress is too firm then a topper can certainly be an effective way to add some additional softness and pressure relief although there is always some risk and uncertainty involved in adding a topper if you haven’t tested the combination in person each person is unique and the specifics of the mattress and topper itself along with your own body type, sleeping position, preferences, and what you are used to sleeping on can affect which specific topper would be a suitable choice for any particular person on any specific mattress.
I would be very cautious about using other people’s reviews or experiences on a topper (either positive or negative) as a reliable source of information or guidance about how suitable or how soft or firm it may feel to you and in many cases they can be more misleading than helpful because a mattress/topper combination that would be a perfect choice for one person may not work out nearly as well for someone else (see post #13 here).
Having said all that … the most likely reason for shoulder pain is a mattress that is too firm and a memory foam topper would certainly add some additional softness and pressure relief to your mattress so the odds are in your favor but again the only way to know whether you will sleep well on the combination will be based on your own personal experience.
It’s also possible that shoulder pain can be a pillow issue and it may also be worthwhile trying a different pillow that keeps your head and neck in good alignment to see if it helps as well…
I’m looking forward to finding out how the topper works out for you.
Phoenix