1. Yoshi can't crawl.
Look at his physique.
He doesn't have the natural build to properly "crawl". He crawls in Brawl, but it looks completely unnatural and unorthodox.
It turns from immobility to poor attempt at crawling.
His arms are too frail looking to give him enough pull and his legs would barely allow for much push. It turns more into an attempt to push himself foward rather than walk on all fours.
All fours doesn't work.
Take a T-Rex, beef his arms a tad bit, severely shorten his legs, shorten the tail, enlarge the head.
You got Yoshi.
His arms wouldn't even be enough to properly support the weight of his entire head.
Crawling is useless. Especially in a fight or flight situation. He's literally moving like a slug or not at all.
2. He can't tackle
He can't run due to pathetic legs.
He can't walk due to pathetic legs.
He can't stand due to pathetic legs.
He can't crawl due to unabiding build.
He lacks mobility.
A tackle is nigh impossible.
3. He can't tail whip. His tail is far too short to do any damage.
He'd have to spin his entire body to get a decent hit off, but as said before, he significantly lacks the ability to move.
4. Yoshi's bite could potentially have some affect, but it is extremely easy to avoid and get out of.
a. Yoshi can barely lift his head from the ground. Luigi simply walks around him.
b. If ,for some ill forsaken reason, Luigi stands directly in front of Yoshi and he manages a bite, Yoshi has the largest set of eyes that are begging to be mutilated.
It's more of a lose chance for Yoshi to bite as Luigi makes mincemeat of Yoshi's eyes via the vacuum (nozzle end). Yoshi ends up blind while Luigi ends up with a few minor cuts. Omnivore or Herbivore. It won't do that much. Especially since his arm or leg would be the only viable "bite choice" Yoshi has.
5. Yoshi is too topsy turvey to actually stand.
Too much of his weight is situated in his front body/neck area.
Just standing would be a fight to keep proper balance from tipping over.
Even on his belly, it is too much weight to keep his head up. Perhaps for a couple of moments, but not long at all.
6. Luigi has (going souly by what was mentioned) a vacuum and his own brute strength.
Don't understimate Luigi. He's still a human that's in his mid/late-20's.
Accompanied by a makeshift weapon.
He can use the nozzle end as a stabbing weapon or use the bulk/storage unit as a blunt force weapon.
a. Repeated stabs at Yoshi's eyes, back of neck, hide.
b. Use the unit as a blunt force to the back of Yoshi's head. Repeat if not successful with first blow.
7. It is not like the Galleom VS Petey MU
Petey had no method of movement.
Galleom had no method of movement, but he had rockets.
Yoshi has no method of movement, but has a bite that's easily countered.
Luigi can freely move about and has a makeshift weapon to damage Yoshi.
8. Yoshi can't turn in any significant manner.
He can only shift his body weight to turn, but it won't be fast or effective against an opponent that can walk around him.
9. ~Desu