Slime is not only the mascot of the series(unlike Goomba), but also has multiple forms and abilities(which Goomba barely has), and stars in its own games(again, unlike Goomba).
The comparison falls flat entirely. Slime is highly significant however. He's the mascot not just cause he's cute, but he also inspired the entire archtype of enemies, and has well over 20 different forms. Actually, way more than that. It's one of the most diverse races in gaming history, with no one in Mario coming close bar Mario himself at best. Goomba doesn't have that many either, and only a few are kind of similar to what Slime has. It's a bad comparison. Besides that, Goomba didn't inspire an entire archtype of enemies to be used in thousands of games, now did it? It's just popular because it's a well-known enemy. That doesn't make it remotely viable as a Smash character, as they need actual abilities to make into a moveset. Slime has a ton of official abilities among its series. Toad... actually doesn't. He has a very lackluster amount of known abilities and does not really stand out as unique in the Mario universe. Slime has unique abilities that other monsters in the series don't use, but more importantly, has a ton of official stuff that only works with the race. Take the Slime Healers. Healing is not that common in DQ for monsters. At least not compared to actual Heroes/Classes using it. For that matter, let's look at the Metal Slime. It's what inspired the "becoming Metal make you near invincible" type of thing. Metal Mario is pretty much the same idea. You don't see Metallize as a common ability, but you do see it as a hard power-up or a separate character instead. Those are just two examples of enemies within the Slime family that are fairly unique or helped inspired RPG's to come.
Slime is the de facto "first enemy" and pushed into other games to represent Dragon Quest. Goomba? Definitely not in the same way. He's got the first enemy bit down, but let's not pretend he's put in much of anything. Or has enough actual abilities to make a moveset. Or does much that super stands out on its own(what, you think headbonking is that unique? It isn't). We also don't a liquidy fighter either, so Slime already stands out way more than Goomba in comparison.
