They might be able to beat some of the lower-tier superhero groups out there, and it would have to be lucky. Groups like the Young Avengers or the Runaways or some of the second-string X groups (provided this is a pre-decimation meeting), but even that's a bit of a stretch. Sure they could morph bugs and hide, or morph something big and cause a lot of damage, but eventually it's gotta come down to a fight...and honestly there's a lot of superheroes they could beat in a straight physical fight, but in a team setting they're going to be outclassed. While the Animorphs have access to flight, inhuman strength, speed, and durability (plus the morph healing factor), other heroes have access to either much more intense versions of the same powers or powers that go beyond - control over elements, control over materials, mental powers, the list goes on and on. Even with Ax, there is just a little too much for them to handle, I think. If they went up against some of the heavy-hitting teams out there they would just be crushed. Avengers? Defenders? X-Men? X-Force? Alpha Flight? Excalibur? Fantastic Four? There's no way. I think it's unfair to make the comparison, honestly.
Could they beat Captain America? I'm gonna have to say nay-nay. With his level of experience and ability, I can't see it. But Cap's also been my favorite superhero since I can remember, so I think that may have something to do with that decision
Besides, I'm fairly sure Cap wouldn't want to fight them. After all, they're doing a pretty good job (for the most part) doing the right thing.
