It's a huge issue; it's the issue for which I no longer follow baseball seriously. Any sport not played on a level playing field is a farce.
There are rare instances where low-monied teams win, but these are rare. And they tend to be blips, whereas the high-monied teams are competitive routinely.
It is but even if you had a salary cap at Houston Astros level (which you're not getting with the MLB union) there'd still be a $100 million difference than that and the bottom payrolls, there's always been a disparity between the haves and the have-nots in baseball because of revenue differences, as long as there's been a major league baseball and it's not really affecting general popularity. You could say the same about the NBA who arguably is even worse with the have and have-not structure because of the way the sport is, despite a 'soft cap'.
And you do have the luxury tax which makes it pretty costly to spend over a certain amount for multiple years, though it only really affects the top 3-5 teams and they've made some creative accounting moves to get under the tax threshold. Even if you put a cap up teams will find ways around it, spending more on scouting, development, infrastructure, being able to spend to shed cap hits...the money differences will manifest somewhere. A bigger problem for baseball (besides the start time and length of games) is that more and more teams are divebombing into the 'be terrible to be great' mode now that the Astros and Cubs have made it en vogue.