ANSWER: No, leave them out so you don't burn the garlic or overpower the naturally sweet roasted carrots.
The garlic in your lemon butter sauce already brings plenty of flavor without competing against the sweetness of the rainbow carrots and white onion.
Chopped or minced garlic on an open sheet pan risks turning acrid and bitter over the 25-minute total baking time.
If you crave extra garlic, toss in one or two whole unpeeled cloves with the vegetables so they soften into sweet, mellow garlic paste instead of burning.