Is the YBB-105WC good for a beginner?

Yes. Its 3/4 size and three top-action valves make it manageable for students, and Yamaha's intonation is consistent. It is a common first tuba for school programs.

Does it come with a case and mouthpiece?

Yes. It ships with the BBC-27 case and a Yamaha 67 mouthpiece, so no extra purchase is needed to start playing.

It is a BBb tuba, the standard key for most school and concert band tuba parts.

How does it compare with a full-size 4/4 tuba?

It is smaller and lighter with a .661-inch bore, so it is easier to handle but has less low-end power. A 4/4 tuba suits advanced players and larger ensembles better.

Can it play low chromatic notes?

With three valves it covers the standard range, but the lowest chromatic notes are harder than on a four-valve tuba. Beginners rarely need those notes.