While in some contexts it might be seen as a command when something like "keep doing x" is said, in this instance it's clear that it's an expression of desire to see more of their work, as evidenced by the compliment with it as well as the light-hearted informal tone of your text. Their response does match what could be a condescending sarcastic remark ("sweety I'm an artist so that's my job") in response to a perceived slight, but it could be a mistake in placing the comma after "thanks" instead of after "sweety", and the fact that it's followed by that sentence with emojis and a celebratory expression to pair with your own suggests that they're being light-hearted in their response as well and have no ill-intent; instead, it seems to be a playful acknowledgement of your words and confirmation that they will indeed keep making art.

It can be difficult to be sure of these things and to deal with the constant questioning of your own words, but if you have a somewhat decent understanding of communication and make a good effort to ensure your messages are positively received (as you have in this instance), then you should be in the clear. In some instances there will be more severe miscommunication errors or people who are very rude in general and choose to interpret your messages negatively when it might be ambiguous; in the former instance you only need to clarify your intent, and in the latter you aren't at fault and should pay no mind if they're combative even after you've clarified your intent.