Lucas is my main, with Cloud being a close secondary, so i know both characters very well.
When Lucas players use the zair into jab, blocking is your best bet and then going for your jab, because Cloud's is fairly fast as well. Use Cloud's neutral b for pressure but use this rarely because if used often, Lucas can easily use his down b to absorb it.
A lot of Lucas players tend to use short hop nair. To counter this with Cloud, use your own short hop nair. This move for cloud is fast and the hitbox hits a good distance away from him. Lucas's nair's hitbox is very close to him so you will be able to get him away from you.
If Lucas is spacing away from you with pk fire and zair, try to dash to him and power shield when needed. Cloud's dash is fast enough to get in quickly. Power shielding Lucas's pk fire and zair may be hard at first but with time and practice you'll be able to react fast.
Last thing is that Lucas players almost but not always need a grab to guarantee a kill. To not get grabbed try to roll and spot dodge more, but of course don't get too comfortable with this habit because you can of course get punished. There is low lag on Lucas's missed grabs so you'll want to quickly dash in and grab or go for a dash attack or limit charged attack at high enough percents for the kill. However if he is too far away, i would not go for the punish because you may miss and then Lucas can capitalize on that.
Hope this helped and if you have any other questions on the matchup feel free to ask.