SML1 was basically just a proof of concept, it’s so short, lacking graphical detail and proper Mario physics. It’s not awful or anything, but if it didn’t have “Mario” in the title it wouldn’t even be remembered today.
SML2 isn’t perfect but it’s one of the best original Gameboy games, and really shines on its own. They don’t even feel like they’re part of the same series. I highly recommend the SML2 DX hack, played it on my Analogue Pocket and loved it, without the slowdown and with colour it’s very fun.