Using "Free" Sprites in Commercial Games

[Kosheh]

I'm not making a huge game. This is my first real project, it's best not to be too ambitious here. So far I am happy with the sprites I have, they don't seem like placeholders.

WELL

If you're doing it as a "oh, this is my first, small game I made with GM Studio", then it's no big deal. you might even come back to it someday and spruce it up until you feel it meets your expectations.
Yes, even if you put it on your website as part of an online resume, making it with default assets is OK as long as the game is mechanically sound. Also, originality gets you points with potential employers/admirers/etc..

but if your intention from the beginning is to monetize off of it or something, then yeah, you'll want to create your own assets.