No. Understandable, but not right.
Returning evil for evil is not right. The fronds should have been picked up and discarded and then practice either Proverbs 19:11 or Matthew 5:22-24. Since it is likely the neighbor will probably mow other fronds into your yard you may want to go ask he either make an effort not to do so or that he pick up debris placed in your yard - but remember that would entail him crossing the property line coming into your yard. Perhaps the solution is simple: let him know and have an agreement that any fronds found in your yard as a consequence of his mowing will be placed in his yard. If he agrees then you can go back to tossing the fronds back in his yard but you'll still be dealing with fronds left in your yard.
If your neighbor is unaware of his actions, unaware of the offense it causes or just doesn't care will influence his response and the future of your relationship so think about your response with the end is mind. Remember, the neighbor may or may not be a Christian, and if a Christian not a mature Christian, sufficiently equipped to handle either his wrongdoing or your requirements. Don't expect non-Christians to behave like Christians.
I would encourage you to read and meditate on Romans 12:9-21 correctly understanding the cultural idiom of heaping coals on another's head,
Romans 12:9-21
"Let love be genuine. Abhor what is evil; hold fast to what is good. Love one another with brotherly affection. Outdo one another in showing honor. Do not be slothful in zeal, be fervent in spirit, serve the Lord. Rejoice in hope, be patient in tribulation, be constant in prayer. Contribute to the needs of the saints and seek to show hospitality. Bless those who persecute you; bless and do not curse them. Rejoice with those who rejoice, weep with those who weep. Live in harmony with one another. Do not be haughty, but associate with the lowly. Never be wise in your own sight. Repay no one evil for evil, but give thought to do what is honorable in the sight of all. If possible, so far as it depends on you, live peaceably with all. Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, 'Vengeance is mine, I will repay, says the Lord.' To the contrary, 'if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will [rekindle his warmth and sustenance].' Do not be overcome by evil, but overcome evil with good."
Vengeance is God's, not ours. We are called to love. Take him some brownies when you go talk to him. That does not mean to tolerate wrongdoing; it simply means upholding godly standards in godly loving ways. Remember also Gal. 6:8,
Colossians 4:5-6
"Conduct yourselves with wisdom toward outsiders, making the most of the opportunity. Let your speech always be with grace, as though seasoned with salt, so that you will know how you should respond to each person."
You have to live next to the guy. Don't make matters worse. Maybe do a study of the word "neighbor" in scripture and note the best means of having good neighbors is to be a good neighbor.
