Is Mewtwo In Brawl?

No but there is are placement character and he is lucario

Well actually, they were planning to put him in. But it seems that Lucario and Jigglypuff were more favorited then Mewto. I also would like to play with Mewto. I used to play with him in Melee...

Here is the music that they were planning to put into the game, however, they have been scrapped. As you can see, "snd_bgm_Z38_FMYU2 (Mewtwo's Victory Theme)" is in the package.

Quote from e-mpire.com:
Snd_bgm_X12_SIMPLEINTRO (Classic Mode Intro)
snd_bgm_X14_ENDING (Classic Mode Ending)
snd_bgm_X24_HOW2PLAY (How to Play)
snd_bgm_A11_MLRPG02 (Another piece from one of the Mario & Luigi RPGs)
snd_bgm_A12_MORINOKINOKO ("Mori no kinoko" = Mushroom Forest...which game? Could it be...SMRPG?!)
snd_bgm_C06_KAZENOSAKANA (Ballad of the Windfish)
snd_bgm_E04_COOKIE (Yoshi's Cookie)
snd_bgm_G06_COMMAND (Star Fox: Command)
snd_bgm_J01_STAGECHANGE (No idea...maybe BGM for when Castle Siege changes forms?)
snd_bgm_J05_ERABARESHI (Erabareshimono = "The Chosen One(s)")
snd_bgm_K02_SENTOUONIISAN (Battle with Masked Man from MOTHER 3)
snd_bgm_K03_EIGHTMELODIES (Eight Melodies from EarthBound)
snd_bgm_K04_SMILEANDTEARS (Smiles and Tears from EarthBound)
snd_bgm_K06_BECAUSE (Because I Love You from EarthBound)
snd_bgm_M14_WARIOSTAGE (Another Microgame$ tune)
snd_bgm_N04_RADIOTAISO ("Radio Taisou" = Radio Exercises)
snd_bgm_Q03_SPORTSMEDLEY (Soccer / Baseball / etc. On the NES, probably)
snd_bgm_R01_WILDTRACKS (No clue...)
snd_bgm_S01_MAINTHEME (MGS Main Theme...no doubt taken out of the main game because of the composition controversy)
snd_bgm_S09_BEATMANIA (MGS music from Beatmania?)
snd_bgm_T04_HOWTOPLAY (Original "How to Play" music?)
snd_bgm_U05_UCANDO ("You Can Do It"...? Probably another vocal track.)
snd_bgm_W22_DX55 (The missing Melee track...Fourside?)
snd_bgm_Y06_ADV06 (Missing Subspace Emissary music)
snd_bgm_Y12_ADV12 (Missing Subspace Emissary music)
snd_bgm_Y18_ADV18 (Missing Subspace Emissary music)
snd_bgm_Y19_ADV19 (Missing Subspace Emissary music)
snd_bgm_Y20_ADV20 (Missing Subspace Emissary music)
snd_bgm_Y21_ADV21 (Missing Subspace Emissary music)
snd_bgm_Y22_ADV22 (Missing Subspace Emissary music)
snd_bgm_Y23_ADV23 (Missing Subspace Emissary music)
snd_bgm_Y24_ADV24 (Missing Subspace Emissary music)
snd_bgm_Y25_ADV25 (Missing Subspace Emissary music)
snd_bgm_Y26_ADV26 (Missing Subspace Emissary music)
snd_bgm_Y27_ADV27 (Missing Subspace Emissary music)
snd_bgm_Y28_ADV28 (Missing Subspace Emissary music)
snd_bgm_Y29_ADV29 (Missing Subspace Emissary music)
snd_bgm_Y30_ADV30 (Missing Subspace Emissary music)
snd_bgm_Z09_FLUIGI (Luigi's Victory Theme)
snd_bgm_Z12_FKUOPA (Bowser's Victory Theme)
snd_bgm_Z13_FPEACH (Peach's Victory Theme)
snd_bgm_Z14_FZELDA (Zelda's Victory Theme)
snd_bgm_Z15_FSHEIK (Sheik's Victory Theme)
snd_bgm_Z19_FFALCO (Falco's Victory Theme)
snd_bgm_Z20_FGANON (Ganondorf's Victory Theme)
snd_bgm_Z24_FZEROSAMUS (ZSS's Victory Theme)
snd_bgm_Z26_FLUCAS (Lucas' Victory Theme)
snd_bgm_Z27_FDIDDY (Diddy's Victory Theme)
snd_bgm_Z28_FPOKETRAINER (Pokemon Trainer's Victory Theme)
snd_bgm_Z32_FDEDEDE (King Dedede's Victory Theme)
snd_bgm_Z33_FLICARIO (Lucario's Victory Theme)
snd_bgm_Z34_FIKE (Ike's Victory Theme)
snd_bgm_Z37_FPURIN (Jigglypuff's Victory Theme)
snd_bgm_Z38_FMYU2 (Mewtwo's Victory Theme)
snd_bgm_Z39_FROY (Roy's Victory Theme)
snd_bgm_Z41_FTOONLINK (Toon Link's Victory Theme)
snd_bgm_Z52_JCLEAR1 (Some accomplishment music...probably for clearing a stage / event)
snd_bgm_Z53_JCLEAR2 (More accomplishment music)