You can go ahead and mulch now, but just keep the mulch back a tiny bit from the plants so it doesn't smother them or interfere in their growth. Also, I've noticed very early in the season that pill bugs, sow bugs and other soil-dwelling pests hide in the mulch while eating young plants, so I usually keep the mulch back about an inch away from the tiny plants so the pests cannot hide in the mulch while nibbling stems. One the plants are taller and have thicker stems, you can push the mulch in more closely.