Mom died, so I planted mums and forget-me-nots all over her grave site.
Grave Flowers
Explanation
Explain Bear
Listen up, chucklehead! This "joke" is just playing with words. Mums are flowers, see? And forget-me-nots? Those are flowers too! So, the person planted flowers that sound like what you'd do when someone dies. You probably forget to water your houseplants, don't you?