Before you make a topic/post, consider the following:
-Is there a topic for this already?
-Is your post on-topic/appropriate?
-Are you posting in the right forum/following the forum rules?
-Showcase all of your levels in one topic.
I would make a few basic design changes to your level:
-If you're going to add a ceiling, bring it up a little. Your level looks a little compressed when it has a ceiling, but only in the first section.
-Piranha plants don't need to be and shouldn't be in every single pipe.
-I'd use the waterfall coming from the grass ceiling a little sparingly. it looks fine coming out of the pipes.
-Don't use clear water in a land level. In an inside level, yeah, but I'd just use a solid blue color and then use seaweed if you think it looks empty.
-Lessen your usage of coins.
-That clear water section has gates and trees underwater. That looks strange and honestly kind of stupid
Otherwise it flows nicely. It looks nice, sounds nice. It functions as a fabulous, basic, introductory level. Maybe a little too basic, add a little something to spice it up if you think it'd fit. right now I'd sit a score at about 6/10.
Electric gold our love with tender care
Hills of satin grass and maidens fair
Now she rides through the night on a silver storm
Sword in hand, our fate's torn
She's a sparrow of the dawn
Our love is born
-Flower Power, Greta Van Fleet
Overall, the level was designed pretty well. It managed to be unique and varied without relying heavily on a particular gimmick, and that's no small feat. The stage was designed to accommodate the NPCs in it, which is something I rarely see. However, there are some things I would change, such as the over-abundance of water. There was a lot of it which really didn't belong there, and I don't think the level would be any worse off without it. But, aside from that, it was pretty damn good.
I have no problem with the tilesets and BGOs you chose. They all blended together and felt natural, and, apart from missing water BGOs and waterfall spam at one point, they were placed effectively. My issue lies in the background. The base multi-coloured hill background was a good fit, but I don't think the added castle wall really fits, especially for a first level. The lower half of the background also looked as if it had been shrunk, and it looks noticeably different from the rest. If I were you, I would have just gone with the base background. Also, I don't really think the music complemented the level well. The Open Plane is the kind of track you'd use for a large, open, expansive level with an emphasis on exploration, and Mushroom Passage just isn't that. I think, if you're going with Super Paper Mario music, Lineland Road would be a better fit. At the end of the day, the graphics and music were decent, but I think there are quite a few improvements you could make.
This is my main issue with the level. While I think the coins were placed well, there were just too many enemies, especially for a first level. It was crawling from top to bottom with Goombas, and almost every pipe contained a Piranha Plant. This kind of thing would be OK late-game, but for a first level it's just too much. You want to ease the player in with straightforward levels and gradually build up from there, but this throws you right into the deep end.
Overall Score: 7/10
This level is generally pretty well-designed, and, with a few tweaks, it could become something truly amazing. Looking for more levels from you in the future!