Not at all, there are tons of lighter guys who do MMA. In fact I think lighter fighters are generally way more fun to watch. The weight classes go down to flyweight which is like 57 kg, and everyone gets treated exactly the same regardless of your weight class. You've got nothing to worry about.

I love MMA and BJJ as martial arts and having some knowledge and experience of them can be nothing but good for you, but in terms of street fighting both are lacking in quite a few ways.

They can both make you habitually feel a bit too comfortable with going to the ground and looking for submissions, when in most real life situations that's a very bad idea for quite a few reasons. Being sports martial arts they tend to focus too much on 'winning' a fight, whereas in a street fight you're just looking to control the situation to a point where you can get the fuck out of there. You may think you'd behave differently in a street fight regardless of training but these things some reflexes that are hard to break, especially under pressure. Very little in either martial arts is particularly helpful when dealing with multiple or armed opponents.

The main reason why sports martial arts like BJJ, Kickboxing and MMA aren't that useful in the street is because they just aren't fucking brutal enough. Street fights can be dirty as fuck, so there are tons of techniques you might need to use or have to defend yourself from that just plain aren't accounted for in sport fighting. This includes groin attacks, headbutts, eye gouges, strikes to the back of the head, knee stomps, weapons, incidental weapons, multiple opponents, strikes to the throat, limb destruction etc. These come up a lot in street fights. In any fight the person with the most options generally winse, so if you're in a situation where your opponent is a real threat you have to be ready to defend yourself from dirty fighting and fight back as dirty as they do.