It's good to grab a lot. That means you are taking advantage of Olimar's ranged attacks. However, it seems like you're not fully utilizing Olimar's range. You say that you use a lot down-airs, back-airs, and forward-airs. All of those attacks are fairly powerful, but they require you to be very near your opponent. As a result, you are putting yourself in unnecessary danger. I recommend taking full advantage of Olimar's ranged hit-boxes and attack more often on the ground, when you are able to. Jumping into the air puts Olimar at risk if the opponent approaches you while you're in the air. Staying on the ground is actually safer, because you can protect yourself from more angles.
As for killing, sometimes it's good NOT to rush towards your opponent when they are at high percentages. Your opponent will actually expect you to come towards them, trying to get the killing blow. Therefore, you are making yourself more predictable by trying to obviously kill them as soon as possible. I find it useful to play mind games with your opponent, before trying to actually land a killing blow. Latch a couple of Pikmin onto them. At this point, the Pikmin are not used for damage, since you've already gotten your opponent to a high percentage. The Pikmin are merely used to distract your opponent, so you can approach them and kill them. If your opponent is smart, he or she will ignore the latched Pikmin. If they are already above 130%, taking a little bit more damage is not as important. However, most people succumb to their normal, instinctive tendencies to try to get latched Pikmin off of themselves as fast as they can. This, ultimately, distracts them from Olimar, allowing you to approach them for the killing blow.
A lot of Olimar players like to play defensively in the beginning, then switch to an offensive style when their opponents are at high percentages. On the contrary, I believe this only makes Olimar more predictable. Most people tend to become more aggressive and offensive when their opponents are at high percentages, no matter what character they use. This is true for most people, whether they are using Olimar or not. Therefore, if you become aggressive when your opponent is at high percentages, you are only fulfilling what your opponent has already predicted of you. I encourage you to take your time. Be consistently spacey throughout the battle. Even when you see your opponent is at 130-150%, fight the temptation of becoming aggressive and stay ironically defensive. Be opportunistic, and throw Pikmin for mind games to open up more opportunities.
To sum it up, don't jump too often - and if you do jump, DON'T double jump, and try to do more short-hops. A lot of grabbing is a good thing - keep doing it. Throw Pikmin for mind games when you want to kill your opponent.