Hi Vita
Welcome to the site.
There may be a few reasons why your Meringue Kisses are grainy/bumpy. My first thought is that the sugar hasn't dissolved properly when you have mixed the batch. Having too much or too little sugar affects the structure of the meringue and can give you grainy or bumpy results when baked.
I'm not sure what sugar you have used but make sure you use something like Caster Sugar instead of Granulated, as this has smaller sugar crystals and dissolves easier into the mixture.
I know you said the mixture looked smooth, but a good way to check if your mixture has been incorporated properly is to rub a little between your thumb and forefinger. If the mixture feels grainy, mix for a little longer and then test again, ensuring that your mix feels smooth.
Your temperature sounds okay, so I'm wondering if they might have needed baking for a little longer? If you're happy with the bake time, once baked, always leave the meringues in the oven with the door ajar so that they can fully dry out and acclimatise before removing them. Then store them in an air-tight container.
If you're not happy with the overall look of them but they taste good, you can always crush them up and use them as toppings for ice cream, cupcakes or a deconstructed strawberry shortcake.
I do hope some of the above is of help to you, and hopefully, you'll be whipping up a lovely batch of kisses in no time!